You are incredibely right, I recently buy 2 jeans 514 33/34 ... and they are not the same size as thus bought one year before ... Seriously Levi's are you drunk?
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.
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.