I'm not sure if I've got some sort of mental filter that spots these, or if it's a commonly encountered problem, but this is the third time in a week someone has asked this question.

The answer is that my has both a compile- and a run-time effect. The compile-time effect allocates space for the variable and lets it pass strict. The run-time effect initializes the variable. If you conditionalize the initialization you have, in effect, a static variable; one that retains its value beyond scope exit.

I've explained it in more detail, with a documentation reference, here, and it has been covered in a thread here.