It's doing exactly what you're telling it to - which is something different from what you want.
Do it out by hand (this is what you should always do when you run into an issue like this):
a=10
b=1
c=0
First round:
b=b+1 (1+1=2 - b is now 2)
c=a+b (10+2=12 - c is now 12)
Second round:
b=b+1 (2+1=3 - b is now 3)
c=a+b (10+3 = 13 - c is now 13)
and so on.