Why does the formatting of a Google Document change when I use a different computer?

Text in Google Docs is basically HTML. When a document is opened the web browser formats the text based on the size and font type. 

Each browser renders fonts and pages differently. For example, a 12 point Arial font looks different in Safari, Internet Explorer, Firefox and Chrome. This is why the same text and page breaks will look different across different browsers. This also happens across operating systems, for example Firefox in Microsoft's Windows 7 will render text and pages differently to Firefox in Apple's OSX.

Unfortunately there is nothing that can be done to change this because it’s a side effect of the way browsers work.