There are two possibilities here:
1. You have the same style names with different definitions in the two documents (and this can be very hard to track down if styles are based on other styles). For that situation, style definitionn for the receiving document control the formatting.
2. You are pasting formatted text (even if you don't expect to be). I think we've seen similar reports from other CC 2015 users. Try trashing the prefs. See Replace Your Preferences