I believe its the audit length. The one a day is the long audit. I only did one of these, years ago and said never again, without a substantial increase in fee! I am doing one of the shorter ones this week, after a long absence, so don't know how much they have changed.
The short one is really short, especially once you've done it once or twice -- at that point it can be done in well under 30 minutes. The main factor is how long you have to wait for an associate to be available.
The long audit (1 per day) is auditing "corporate" stores. The short audit (2 per day) is auditing "non-corporate" stores. The long audit also has added sections for questions concerning pay, security alarms, security, etc.
I tried to do a non-corporate shop and they would not allow me to do si, I was asked for ID from MS or from his parent company. I was paid for the shop anyway.
I made an appointment but some supervisor was in the store when I arrived.
Years ago I did one and found them very easy as long as I printed out the form and had a cooperative employee.
I did the short one yesterday. It took me about an hour, but that was because one of the questions which required an auto-generated email response wouldn't auto-generate! It probably would have been closer to 45 minutes if there hadn't been that issue. There was only one customer that came in during that time. I scheduled two more next week because they are heavily bonused at this point. I still won't do the long audit, mainly because I have an eight to five and you can't do them on the weekends. I'd say, go for it, if you know there wouldn't be heavy customer traffic.