Weridness when using += or ++ with uninitialized variables

When dealing with a previously uninitialized variables, the follow **does not work** properly:

_variable += 1_

This also **does not work** properly:

_variable++_

But the following **does work**:

_variable = variable + 1_

And this also **works**:

_variable = 0_ (somewhere during a creation step, not here right before the incrementing as that would "work" but lock it at 1 obviously)
_variable += 1_

Why or how I don't know, I was just trying to get someone elses simple old project to work properly and this change got a lot of stuff working in it.