The problem is, the true-type font's displayed on a users computer depend on which fonts are installed. When designing anything for the web, this needs to be taken into consideration. Take a look at this post at W3Schools to see a list of "Web safe" fonts:
To make sure your fonts are not changing when played on different computers, you will need to either:
1- Choose a true-type font that is 100% compatible and installed by default on all systems your game runs on.
2- Switch to a bitmap font. Since bitmap fonts are stored with the game and loaded at run-time, they are always guaranteed to display properly on all systems.