Two days last week, I had almost identical stats except for the fact that one day I had 49 more miles. My dispatch increased exactly 36 minutes. I asked my center manager how that is possible because even if they were all at 60 mph then my dispatch increase should be 49 minutes. Hell, I don't think anyone can average 30 mph on a route with all the stopping; so realistically my dispatch should have been 98 minutes more.
I was never trained on the flux capacitor in my package car, because the governor shuts it down at 66 mph. Maybe if I could hit that magical 88 mph I could do those 49 miles in 36 minutes.
By the way, the center manager said there must be other variables in there somewhere. Riigghhtt!
Without looking at your timecard to see what the exact stats were on the two days it's hard to give an exact reason, but I can think of a few.
1. Did you deliver virtually the same number of stops in the same areas over the two different days?
2. Did you deliver to different areas or at least slightly different areas on the two days. - Different defined areas have different stop allowances as well as having different to/fr allowances.
3. The biggest factor on time is the on area miles which give you on area time.
In short, the less on area miles you have in relationship to allowed time (from your stops\pkg allowances) the more allowance per mile you get. The more on area miles you have in relationship to allowed time (again stops\pkg allowances) the less allowance per mile you get.
Some people will say a mile is a mile and you should get the same time.
However, consider a country route where you have to drive 1 mile between each stop on average. The driver gets in, accelerates to speed limit and drives the 2 minutes or so (assuming 30 mph) to get to next stop slows down and stops.
Next consider a driver in a dense area where there is 1/5 mile between each stop. The driver gets in, accelerates and shortly thereafter arrives at the next stop. A lot of time spent in acceleration and braking during that 1/5 of a mile. If you put a stopwatch on the driver and had it running only while the driver was driving (including the start time, stop time). The first driver would spend a lot less time driving then the second guy over the same mile. (since the second guy had to start\stop 5 times to only once for the first guy).
Keep in mind, the on road mgmt team wants you to do well and come in scratch, they don't want tighter standards probably more then you don't want it. They get graded on overallowed by their drivers. They have no motivation to make it look worse.