What does it mean when.....?

[67Fleet]

One of my exhaust pipes has a lot more air blowing through it than the other side? My truck has felt like it was lacking power a little before I tore it apart. I'm in the middle of re-doing the chassis, I cut the mufflers out about a month ago, the bed is off it, and it is up on jackstands while I finish my step-notches out back. Today I started it up just to keep the engine lubed and I put my hand behind each pipe. Noticed that the passenger side has a lot more exhaust coming thru than the driver side. Why would this happen? About 2 months before I started tearing it down I did a compression test and everything was within 10psi so I'm totally baffled.