I have just figured the problem out: it was in the "viewport" meta tag, in the home page I used it like this <meta name="viewport" content="width=device-width"> and it worked just fine. I saved this page as and built the second page up on it, but this time I got the problem with the page displaying wrongly in mobile devices (tablets included). I have tried an additional attribute to the tag: <meta name="viewport" content="width=device-width, initial-scale=1">
I made a research and one of the pages of Mozilla, has gave me the answer, which consists in adding another attribute, like this: <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1"> and that worked!
You will find more information about this here: https://developer.mozilla.org/pt-BR/docs/Mozilla/Mobile/Viewport_meta_tag