Hard to say. I had a similar sounding case recently where I found a particular phono preamp setup I was using to convert to digital had a small hardly audible but measurable difference in output levels left to right. It was simply a minor flaw that may have developed with the device over time. I went to a different setup recently but with same cart and step up transformer, and now things measure equal left and right as they should.
So ideally it’s just a poor connection or combo of connections perhaps but always possible any device involved in signal path from the cartridge on could just be off somewhat left versus right. I’ve seen it happen on multiple occasions over the years with phono cartridges in particular.
It’s also possible more than one device in the chain has some l-r output level difference and they cancel each other one way and augment each other to be heard the other.