MP3 Exports at Inexplicably Low Volume

I found some other posts with similar issues, but none of them had a response that worked/applied. Basically, let's say I'm listening to a score's playback at 50% system volume (on my computer). If I export the score as an MP3, I have to bring the system volume up to 80-90% to get the same loudness. I tried increasing the synthesizer volume (from the synthesizer window) before exporting, but that didn't help. I looked through some more of the settings, and I'm out of ideas.