My guess is in the first loop the match happens on the same string each time, so you'll find each a in turn then exit the loop

In the second example, you call InfStr on every cycle of the loop, which makes it a regex against a new string, so it'll find the first a in the string each time and never progress

(Update: You can see this more clearly if you use (.) instead of (a) for the match; first loop will go through each letter, second on will always find the first a)