margin-top vs padding-top

[esseifforte]

Hello, I'm completing a site and I realized that for some pages the height of the footer is not the same, in some that have the margin-top inside a container div, 'eat' that piece of footer. In practice, that part at the top goes down, as in the screenshot. If I change to padding-top it works great. I set everything as indicated.

at this moment I add to 'site-container {display: flow-root;}' and it works..

thanks in advance.