I can think of a couple of reasons: manufacturing tolerance, differently sourced / dyed fabrics (make sure to wash your clothes before first use).

There is also something called vanity sizing (https://en.wikipedia.org/wiki/Vanity_sizing) where sizes go up without the numbers going up, so that people still think they're the same size.

If you're buying clothes a year apart, they're likely using a different supplier using different grades of fabric, differently calibrated machines, so the clothes won't feel or fit the same.

Drunk on cost savings from moving production from factory to factory around the globe.