Not all fonts are equal. And just because you have a font on your computer doesn’t mean I have it on mine. And if you create a document using a font my pc doesn’t have, it will show some other font. So, if you are editing your own website or creating a Word document for people to read via the web, consider this fact and opt for the most common fonts so everything looks more or less the way you intend.
Here’s a good reference to the most common fonts and it shows what similar fonts are called on a Windows machine versus a Mac.