No, I bet there's absolutely no difference between the two buses because I had the same issue (thought it was a very early version of Sonar and they were audio tracks, not buses). The two audio tracks were exactly the same. I contacted tech support only to find no solution. What did I do? I moved clips and other data on those tracks to two NEW tracks and deleted the old ones. Solved. It was a project I'd been working on for months, so I gathered some kind of error happened at one point and made the tracks' output volume different.

So, I think the odds are very good that the OP is missing something. So, try verifying the gain nobs (are they both the same?). The sends to the busses (are they the same?). Disable all FX using the FX bypass button on the control bar (any difference?).

It's possible that you have found a repeatable bug, but just so extremely unlikely. If I had to guess, I'd think there was some chance you had the focus on a control on one of the busses and spun your scroll wheel trying to navigate to another track. You noticed no change in your GUI, but now your busses didn't match.

Another tip, if you busses are indeed identical and without any FX that introduce random variables, you should be able to flip the phase of one of the two and have them cancel 100% to -inf type silence. If you have FX that generate randomness on the two channels (I think console emulators may introduce this randomness preventing perfect cancellation after the phase flip), then the result of the phase flip will be only the differences. You could then bounce them to a new track and normalize to get a level to hear the difference. 

I'm amazed posters here don't realize how easy it is to check the two buses are exactly the same, and how easy it is to fix the problem.  

I have had this issue as well with both tracks and busses. Finally moved clips and deleted the tracks to "fix" the problem.

@Bob

Have you tired inserting 2 completely brand new busses, route your track to them one at a time & checked again?

vaultwit

Mooch4056


Nods head. I see. 

Then I am afraid the only other explanation is its just as much as mystery of as how exactly did those Egyptians build those pyramids. 

I concur. I've resorted to just lowering the volume of the louder bus to make it the same as the other... sigh.

Kentucky windage.... works every time.

What are the inputs you're using? Are they stereo or mono?

Are there any tiny "+" signs next to the volume and pan controls on all the tracks and buses?

Maybe I was a bit cryptic earlier.  I was alluding to the possibility that the OP may have unwittingly switched to "Offset Mode" by accidentally hitting the "O" key.  It accounts for a lot of cases of unexpected volume decrease.