there are a number of different reasons. the dior cuts are named such that they represent the leg opening of a size 32 pair, and they go +/- 0.5 centimeters for every size you go up/down, i.e a size 31 19cm cut should have a 18.5cm hem, a size 33 should have a 19.5cm hem, etc. any other differences are because the length has been shortened (higher up on the jean is wider because they're tapered, hence the wider leg opening), the hem has been stretched (if they've been worn over bulkier shoes, for example), or they've been tapered further after the fact (leading to a smaller leg opening).
either that, or the person posting the measurements has no idea how to measure/are wrong about their pair of jeans being a 19cm cut (I've seen instances of people thinking jeans are 19cm, but are actually 21cm because the person they bought them from was clueless). the cuts have not changed from season to season.