Here is something I always found to be really weird. This experience has been 100% consistent in my decades of working on bikes.

Install brand new chain. When adjusting it, I find no inconsistencies in slack/tension as I rotate the crank. After the first ride there is a lot more slack and there are BIG differences between loose and tight spots.

1. I aint strong enough to stretch a metal chain.
2. The axle didn't move in the dropouts. I checked.
3. WTF?

The loose/tight spots have never bothered me, but the mystery has always caused me to scratch my head.