strange behavier during debuging.

class xxx
{
...
private a;
private b;
private function()
{
...
b = false;
...
}
...
}

I searched the whole project for variable a and put a break point at each place. then when I debut and step through the "b=false;", the value of a got changed in the watch window. I mean just before the statement, a is empty and right after the statement a got some value. program never break else where.

can anybody explain?