I'm going to expand my comment on Frosteeze's answer into my own answer.
Servants have special skills and abilities by virtue of their class. These are granted to them by the grail system, and may give them abilities and skills they did not and could not have possessed in their actual lives/legends. In-universe this is to help bridge the (vast) cultural and linguistic gaps between summoner and Servant, and to make sure a Servant's capacities aren't ruined because they're scared of the demonic picture box, or can't conduct themselves in society even in the simplest of ways. Out of universe, this was probably just to make the writing easier and the characters more relatable to the reader.
On the flip side, the class can also restrict the servant: Cu Chulainn would have had greater use of his rune magic as a Caster class, Hercules would have had access to several of his NPs in any class other than Berserker (and would have probably been more powerful as an Archer), etc. To go on a tangent into the more complicated interactions, Saber's legend includes a mythical spear, so you might expect her to be able to be summoned as Lancer, but due to the overwhelming association of her to Excalibur (both in her own mind and in legends), she can only ever be summoned as Saber.
In Fate/Zero we see several instances of this from Saber.
The first is when she is boarding a plane to Japan with Irisviel. She explains to Iri that she is aware of what a plane is and is not alarmed by it. The grail provides her with this information to give her the fundamentals of the time and area of the war she is summoned for. This is also why she and every other Servant speaks fluent Japanese, incidentally. Or why Rider has no issues with reading Japanese texts, despite it being questionable at best if she would have even been literate in any language in her myth. But more importantly is what she says immediately after that. While she knows what a plane is, she has no idea exactly how it works or even how one would fly it. Nevertheless, she knows that as soon as she sits in the pilot seat she would would have zero problems flying it expertly. This is due to her Ride skill: it lets her "ride" or "pilot" anything up to a certain degree of mystery, and her rank B in particular easily includes non-magical transports such as planes and motorcycles. She also expertly rides a super-charged performance motorcycle later on in Fate/Zero.
The Saber class also has intrinsic sword skills, and they will work in the same way: the grail will provide them knowledge of how to use their "sword" up to the proficiency level dictated by their skill. In the vast majority of cases (except, say, Zero Lancelot), a servant would only ever use their Noble Phantasms or other weapons that came with them at the summoning. As such, while Saber probably was not provided with intrinsic knowledge of actual kendo combat--though as part of the generic cultural information she probably knew what it was in basic terms, what a shinai looked like, etc.--, she was granted expert level proficiency with the style from simply holding a shinai.
Fake Assassin (and even True Assassin) also has weapon skills, and in Fake Assassin's case they exceed Saber's. You might note how well Fake Assassin handles Saber, despite her fighting in a foreign style with an obfuscated weapon. And similarly for how Saber handles a foreign opponent with what should be an unfamiliar style. This is probably part of the standard trope of legendary fighters being able to read their opponents and know their fighting style after just a few exchanges, and would be encapsulated by the weapon skill involved.
