Element exists activity output variables in pick activity

I find that if I assigned a Boolean variable A as false and in pick activity I use element exist activity output this Boolean variable A, even if this element exists, the Boolean variable A unable to change to true.

but if I add a new Boolean variable B, take it in element exists activity, and let A=B, if element exists, B will go true and as well as A.

Can anyone explain why the variable unable to change if I use only one variable, but it is effective when I use 2 variables?

Can you send any visual flow example to illustrate this?