Block chance Reduction Bug?

Hi. I have noticed one thing that worries me, idk if its Bug or what ? I went for max block on my pally, and every time i lvl-up i lose 1% chance to block. got 75% all the time, and after each lvl-up it goes down to 74% so i have to invest 2 points in dex to get 75% back, when id liketo invest that in to vita. Whats up with this block reduction ?

How the calculation works, have to keep topping it up as you level with a point to dex here and there