I have to admit Microsoft Word is my bête noire when it comes to computer software and I have very strong feelings about how text styles are handled in this application. As far as I’m concerned it’s only thanks to Maggie Secara that it’s possible to manage Word styles in the long-term. At some point a document will go bad.
Look at what happens each time you copy a small amount of text from one document to another. You beautifully crafted document with a few carefully chosen styles inherits all the styles needed for the text you are adding. That is why every computer needs a utility to just paste plain text.
As this is Word the situation is not quite as simple as this. If you copy over text and the style already exists in the new receiving document, the existing style in the receiving document is used, not modified. This is actually quite logical but mean sometimes when text is copied over it will look the same at the original document and other times it will take on new attributes when it is pasted in. The best solution is to always paste in the plain text when going from one document to another.
Edit: Word documents can be protected to restrict the formatting of a document and prevent users (or yourself!) from applying styles that aren’t explicitly made available. This can also prevent users from applying formatting directly to text, such as bulleted or numbered lists or font characteristics. This prevents document styles being copied over from the source document when doing a basic paste. The downsides are that you lose keyboard shortcuts for text modification commands and embedded objects need to be handled with some caution.