Arm wrestling vs weight lifting

Question

S03E01, specifically Chapter 117

In the post credits scene we see...

How does Yu (Ishigami) win arm wrestling despite that e was previously outdone by Chika (Fujiwara) in weight lifting?

Arm wrestling:

Weight lifting:

Answer 1

Update: Skip to the manga section, as it pretty much answers this. Ishigami is officially stronger than Fujiwara according to the numbers. The rest of the answer still has value in my opinion though, as we can't be sure how reliable or meaningful those numbers are.

It's difficult to say for certain, but there are some factors in play here that would give Ishigami an edge.

1. Fujiwara is demoralized from having been caught cheating. During that bout, Ishigami even recorded her cheating and taunted her with it. This could affect her ability to battle. Additionally, Ishigami could leverage this during their match for psychological warfare. At the very least, having been alerted to her trickery, he'll be watching her carefully to make sure she doesn't try to cheat again.
2. Fujiwara is tired after two matches, while Ishigami has only had one. Ishigami also lost quite quickly after struggling in vain against the immovable Kaguya. Fujiwara on the other hand, battled both Ino and Shirogane.
3. As a male, Ishigami should have superior natural strength to Fujiwara. It's unclear what his strength level is, with the show only saying he's weaker than most boys his age, so we can't say for certain whether he's weaker than most girls his age or not. And considering Fujiwara tried to cheat, I'm not certain how seriously we should take her boasts at the beginning about her strength training. Though they were probably only exaggerations at worse, since she seems to be able to carry the stuff more easily than Ishigami. At any rate, while it would seem Fujiwara is probably stronger than Ishigami, it might not be by much.

Also consider that arm wrestling is not entirely about strength, so it's possible Ishigami could find some way of winning. Even with regard to the strength part, it might depend on which muscles Fujiwara has been training whether she gets any kind of advantage.

See for example: https://www.coachmag.co.uk/exercises/sport-workouts/1552/win-armwrestling

Karmic Reasons

Another point to consider, is the author may have simply sided against Fujiwara and decided she should lose to Ishigami. Fujiwara mouths off to Ishigami about how weak he is, and on top of that, she tries to cheat during the competition. The author might decide that Ishigami should win this match for ethical reasons. Personally, I'm not fond of out-of-universe reasoning for these kinds of questions, though I suppose you could also consider it to be karma.

There were also questions of sexism with regard to this contest, but since Kaguya has already won for the girls, this makes it ethically acceptable to punish Fujiwara regardless. Though I might be overthinking it, and the author may not have even been thinking along these lines. One might also argue Ishigami needed a karmic comeuppance as well due to his behavior and comments, but he already received it when defeated by Kaguya (additionally, the manga gives him a further chastisement, writing "A male who's happy to defeat a female" over his head when he defeats Fujiwara, as if to say he should probably be a little ashamed of himself.

Manga Version

After consulting the manga, which I acquired from my local library, I have made an important discovery. The manga actually includes some tidbits at the end which very much confirm or support some of my points. Mainly, a list of power levels of the characters measured in MP (muscle points). But my other points are also strengthened as well.

1. Boys are naturally stronger than girls. This is more pronounced in the manga, with Ishigami saying, "Anyway, I naturally have more muscle than you, Fujiwara..." The chart at the end lists the average male MP as 60 while the average female is 40.
2. Ishigami is not weaker than the average girl. His MP is 55. While his MP is lower than the male average, it is still much higher than the female average.
3. Fujiwara and Ishigami are at similar strength levels. Unlike Kaguya, Fujiwara did not overcome the 60 MP average of the boys, though she did increase her strength quite admirably at 54 MP. This puts her at a lower strength level than Ishigami, but only by one point, so they should be at similar strength. When considering her lifting the table at the beginning, we should probably consider she only carried it for a few seconds, while Ishigami probably carried it for a considerable distance.
4. The battle is framed as boys versus girls. I feel this is more pronounced in the manga. Ishigami states, "There's no way women and children can beat me!"