When a phpbb user is created, it doesn't create for WP...
i would add: at same time.
(it is, i was supposing, quite well explained on help install, on linked post, and in several posts and elsewhere).
When user register in phpBB side, it is not created at same time into WP. It is added in WP, in this case, when user visits WP side as phpBB logged in, or login WP side first time.
I cannot use more clear words than these to let understand the joke with my Eng.
... how to turn easy things into complicated things ...
this the solution to this issue that will be applied in this way (linking another time this topic):
When user register in phpBB, can be added/registered at same time into WordPress?