Matteo,
in general you are right, the setting enables a head bc with max Q = 0. This BC is applied iteratively, as setting the bc at one node may influence the neighboring ones. To avoid endless iteration, though, there's a max. number of loops - if I remember correctly this is 30. Whatever nodes remain not matching the conditions after this number of iterations can show a higher head.
Hope this helps to understand what's going on.
Cheers,
Peter