I have a macro designed to remove all styles from the text on the system clipboard. I have used both of the techniques shown below, and they seem to work nearly identically, but I've found some situations where I copy white colored text, run the macro, then paste it and the color is still set to white as opposed to just plain text (which would paste as black).
Without context it is hard to know what is going on.
The clipboard, after either of those operations, has only unstyled plain text entries.
Clipboard Caching by the target application
Paste matching the existing style. Technically, when displayed, there is no such thing as plain text, since the text displayed always has some style, and when you paste unstyled plain text it will typically adopt the style of the character that was before it which can be the character that was deleted and no longer exists, since the Mac text system is smart enough to remember what the style at the insertion point is, even if the characters have been deleted.
As for why GetPlainText behaves different, I have no idea since I don't know what it does. Maybe it activates (thwarting clipboard caching) or maybe it actively styles the text in a plain way.