when it gioes from -7.something to -6.something the AOA is INCREASING - it's becoming more positive. Exactly as you expect. You can't just look at the magnitude.
Basically, there will be a conversion factor from vane AoA to 'body' AoA (AoA measured relative to the aircraft reference). By the sounds of it, at +2.5 body AoA the vanes read -7.5 Vane AoA. If I had to guess, I'd say that the vane reading probably changes by more then the body AoA for any change - let's assume a 2:1 ratio.
Then we have:
Body AoA=(Vane AoA+12.5)/2
So, for vane = -7.5, body = 2.5
and, for vane=-6.5, body= 3.0
and, so on.
(Incidentally, while the 2:1 is a guess, our aircraft have 3:2 ratios so its possible. What I don't understand is why they're bothering to display a confusing vane angle to the crew - it'd be easier to display the converted body value...but as you all know, I don't like displaying AoAs, and this is yet another example of why.)