Levels Change when copy and pasting clip to new project

Greetings.

So I have one project — call it X — that has a number of different audio clips/stories sequentially aligned on different tracks.

I have another project — call it Y — that has the same thing — only different clips.

I wanted to try to combine the two. So I copied the clips from Project X and pasted them at the end of Project Y. 

Simple. Everything worked fine. Only I noticed that the clips brought over are now are at a higher level than they were before (perhaps +3db). 

But I've got no idea why. The levels on the tracks themselves are exactly the same. There are no effects on the tracks themselves. And it makes no difference if they're pasted onto mono tracks or stereo tracks.  Somehow the audio is being changed/converted. 

I'm not wildly experienced with sound, so I may be missing something profoundly obvious. But any help is appreciated.