Channel imbalance question

Hi - Hoping a fellow audiogoner can help. I noticed a slight channel imbalance in my set-up. Azimuth is dialed in with a Fozgometer. While trying to track the issue down I reversed the left and right inputs on my phono stage putting the left channel from the tonearm lead into the right input. The imbalance went away. Any ideas on what would cause this would be appreciated.

Thanks