There are 'book times' for jobs like clutch changes. i.e. the manufacturer (either of the vehicle or of the replacement clutch) has put down in black and white how long they think the job should take.
Some garages charge for the 'book time' irrespective of how long it took them to complete the task. (That works to your advantage if they have to struggle but works to the garage's advantage if they get the job done very quickly). Others might use the book time as a maximum figure but (generously) only charge you for the actual time if they can do the job a bit quicker.
So it's possible that the 'book time' for the job is 5½ hours but that (possibly because they'd got the most experienced guy on the job) they completed the task in just 4 hours on the first occasion.
When I had a clutch replaced on a Citroen BX, the garage made a point of showing me the 'book time' before they started, as they didn't want me to complain when I got charged for 8½ hours labour. (The whole suspension system has to come out to get at the clutch on a BX). It was only after they'd charged me mega-bucks for all that work that they found that they'd misdiagnosed the fault in the first place and that there had been nothing at all wrong with the original clutch! (It was the last time that I used that garage!)