[quote=@Lonewolf685]
<Snipped quote by Hantu Kongkek>

Because Sasaki couldn't pass of as Saber. EMIYA was clearly an Archer when he pulled out twin swords. Medusa is obviously not an Assassin.
[/quote]

I'd say those are pretty important outliers. For the most part, Servant classes (at least ones not meant to be vague for plot reasons) are the Class they most resemble. It's a leap in logic that carries plenty of risk, but it's still pretty safe when you're going for an obvious solution. Like, say, assuming that the chivalrous knight in cavalry armor is a chivalrous and knightly Rider.