buffer page lock vs tuple lock (?)

Hi,
if I get it right a heap buffer page (that contains several tuples) requires a
pin and a lock to access the page content, and a change to a tuple requires a
lock on the tuple too. Disregarding some cases (tuple shuffling within the
same page and vacuum) does not this double locking be too much? I mean, does
not suffice to pin the page and lock the tuple (e.g., reading)?
Can anybody help me understanding?

Thanks,
Luca