Why is it more expensive to give a second charged attack to Eevee than to *some* of its evolutions ?

I just noticed that giving a new charged move to Eevee costed 75 candy. I checked Sylveon and it was only 50, but when I checked Jolteon it also said 75 candy.
The 50 candy cost only appeared on Sylveon, Glaceon and Leafeon

(Btw I had never noticed EEVEE needed 75 candy for its second move 😅)