Is it real or just my imagination?

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 .

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

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:)

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.

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.

Is it real or just my imagination?

Don't know. But there's one way to be (fairly) certain, although it will require some investment of time and effort. Go back and forth a couple of times between the two settings. And be sure that the warmup status of everything in the system is the same or similar for each of the assessments.

Regards,
-- Al

I currently and have for about 4 years run two different set of amps in my system the first being a pair of VTL M125s and the second a stereo Canary CA 306, the VTL amps have a recommended bias between 275 and 300 and they don't seem to differ at any setting in that area but the Canary is absolutely sensitive to the final setting, it is recommended to be set between 155 and 165 and I found my sweet spot at 162. Tooblue.