Word processors like Word are generally designed to produce documents to be printed. When Word displays a document in a print layout or page view, it uses the characteristics of the currently-selected printer to determine what the document will look like when printed.
Printer characteristics vary a lot. Default margins, paper size, and other differences in both capability and configuration can make a document appear very differently when viewed or printed on one system as compared to another.
Another common difference is fonts, which are not the same across systems.
If you create a document using one font that happens to be installed on your computer, and then view it on another system where the font is not present, things will look different. Word will substitute something “close” to the font you wanted. Unfortunately, “close” is vague, and can be startlingly different from what you intended.