The likeliest explanation is a lost neutralaffecting several homes. This is a red alert! You should call your power company and report a power outage (seriously).
The interesting thing about yours is that it doesn't move much. Normally they move a LOT when you add and remove 120V loads. That suggests it is affecting several houses and they are averaging.
Why aren't your neighbors up in arms? Trust me - they don't know. When our entire complex was stricken, It took me days to notice, and only when my sweetie apologized for having to put the toast down several times. It took another day for me to get motivated to buy a voltmeter (my meters were elsewhere), at which point Pull mains, pound the alarm, power company is out in 30 minutes, done, happy toast. Meanwhile other tenants had major appliances fry, so lucky us. Anyway, you noticed, which makes you the hero.
But you checked all your circuits! Meaning two. Those have a 50/50 chance of being on the same pole, so giving the same reading. My whole apartment was on the same pole, just found that out that day.
But you checked circuits on opposite sides! Oh really. Pushmatic is the only panel with L1 on the left and L2 on the right. On all others, panel phasing is weird as heck, and you definitely do not understand it, unless you do. So revisit that assumption.