Yamaha MSP5a active monitor speaker pair not matching...

[Farringdon]

It's taken me a while to realise this but the right speaker of my pair of MSP5a monitors is a bit louder and has a more bass level/definition compared to the left speaker.  I've set the volume knobs at the front at the same level on both speakers, and I've tried different volumes but the right side is marginally louder and has more low end.  The difference is small but considering these are monitor speakers it's obviously an issue.

Is it normal for a pair of these active monitor speakers to not match?  The serial numbers are pretty close, 01137 and 01098.  Any tips on how to fix this?  It's no longer under warranty, they've now released a new version called "MSP5 Studio" which looks almost the same....

Thanks in advance.