Without knowing how you set your monitoring, bad cable, connection, or what you might be trying to compensate to, the simple solve here is if your master left and right outs, are effectively even, and you've proofed this out, then you should have no difference in your mix elsewhere. That said, Im not saying it will translate (level and EQ, comparatively, the right way). It likely won't. But the overall left and right will exhibit similar dB output levels.
Asking why here, while well intentioned, is not going to help. Check metering and look at SPL, in your listening space. And elsewhere.
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Just adding one more troubleshooting possibility. Typically, the shortest path is the most reliable method. Try this.
If you feel its invariably your drum tracks, set a simple down the middle pan of them both with the same levels left and right. Send that to your flash drive to play elsewhere.
If it now sounds bad, there may be some impedance issue as to how you're computer sounds, possibly from your audio interface that just isnt the same on other systems. Likely resulting in distortion or bad left/right levels. Typically from the attack (initial strike of the drums). So fast attacks and such might break up, versus what you're hearing from your computer. You'll need to try different ways to monitor, be it some other headphones, different monitors, alternate outs, or connectors. This really isnt a Studio One issue, but a me need to fix, my issue. ; )
Hope this helps.