Is it real or just my imagination?

I don't think it's your imagination. I have an ARC VSi60 integrated amp and about a month ago I started to notice my soundstage overall leaned ever so slightly to the left. I checked everything, the recordings, the connections. I confirmed it wasn't a problem with the speakers, as the problem changed sides when I switched the right and left speaker cables. I couldn't think of how to narrow down the issue to the amp or to my dac or source, so I lived with it for a few weeks until I rebiased my amp. The right channel tubes had drifted from spec a bit, at 58.4 and 58.9 while the left channel tubes were close to spec at 59.9 and 60.1. I set everything back to 60.0 and everything is centered again.

Using imagination is fun.
Descrepancies in few single millivolts should not show drastical changes, but just like Al said you can 'improvise' to reach the sweetest spot of bias alais from specified in manual by experimenting updowns just like you would do by analogy with stylus VTA/VTF settings.

After all your amp shows good build quality and design. Tubes being able to hold bias within tolerance boundaries after 500hours and more likely to stay strong after another 300.

The op is exactly on target in his 'enlightening discovery'. It's not his imagination. What would further 'knock his socks off', after finding the 'sweet spot', is to match the left channel to the right while listening:)

Doesn't really matter if it's real or not- you're enjoying the music, right?

Many tube freaks , well me anyway, bias their tube amps on a weekly basis.
About time for my Response Audio Bella Extreme to go into Winter aux heating service .