I mean is there a reason for it? or is it just to make the gameplay easier? or is there some other reason behind it?

They need to balance it.

I'm guessing people complaining that vampirism is too hard for fire damage. Same with no stage 4 attacks.
Personally I find it stupid. I don't think it needed to be reduced that much.

Either that or Beth changed it for balance, as said above.

Maybe because they are a different clan?

They arent a different clan, they are just blessed by Molag bal. All Vamps in Skyrim are "Volkihar" in-game and are diluted by inferior sires, while Harkon''s court is small and closer to pure