Specifically, there are parts of the wash cycle that are indeterminate. How long will it take to get all the clothes wet with water - well that will depend on the water pressure and how many clothes are inside. How long will it take for the water to heat - Well that will depend on how hot it was when it came in and the grid AC voltage variation. How long will it take to spin - well that will depend how many times it needs to accelerate and decelerate to redistribute clothes to find an arrangement to get sufficiently low vibration.
So... With all these unknowns, the machine simply gives a kinda long estimate, and shortens or lengthens other parts of the washing cycle to 'catch up' with the claim. And if some process takes far too long, it will just overrun. And if you only put one pair of pants in, then it may cut the time back a bit when it weighs it and notices.
Washing machine software is now pretty complex!