It does not. Shift and weekend work are specifically mentioned.
172 hrs seems standard - 40 hrs per week for 4.3 weeks (or 42.5 for 4 weeks etc). Look before quitting for something like this specify that you thought an 80% job would be 137.5 hrs distributed over 4 days and see what happens?
Might be some mistake by someone who forgot you were part time? Rules and regulations dictate how this is managed, you cannot work more than a certain amount of hours on a regular basis.
Why not just call your future employer and ask? It can be a mistake as Kri mentioned.
I will write to the employer and ask for clarification.
There is still 12 days before the start date.
For how many hours are you scheduled? You mention days, but not hours.