thanks a lot for your answer, in the meanwhile I had figured out it had something to do with my single-portfolio.php custom code and fixed it.
I had erased the code
<?php get_footer(); ?> in single-portfolio.php and I've realised this had caused problem. I put the php code back and gave
display: none to footer for the single portfolio post in style.css. this made the trick.
I am afraid I can't explain exaclty why, maybe is about some floating/clearing issue? anyway, mentioning my solution in case it can help anybody .