yeah, that kinda irritates me too.
But you know what's makes it even more confusing? In the past, the max it would show for that first number (the one in blue) was .625, regardless of if it was your base or not. So if you were, say, master yi, with a base AS of .679, it would display, at level 1, .625 + .54. Actual AS gains were based on the .679, but the tooltip implied his base was .625 when it was not. For slower champs, like (at the time) Nunu, who had around .560 or so, it would show the proper .560 in blue as the first number. But when you gained AS, it would add on to that blue first number first (despite the fact that you weren't adding base AS), then anything beyond .625 would be tacked onto the green number.
The confusing part is that, at some point, they fixed that tooltip to be what it currently is... but for some reason left the bonus AS to be added into the first number, which is supposed to be base AS. So they already looked at the tooltip and changed it in the past... but didn't seem to think anything of this oddity