This is a good question. Many things affect planned time; however, one thing that absolutely doesn't is the driver on a route. I did a quick "back of the napkin" calculation using 18 MPH overall and 0.025 hrs/stop for delivery and 0.02 hrs/stop for PU and the standard package allowances for Del and PU and came up with 10.86 planned hours for your day. Since there are other things that go into planned time (start work, finish work, CODs Del Confirmation, etc.), at first glance it looks like your day is a little low. I have seen, though, entire centers running over 20 SPORH, so it is not ridiculous that your particular area plans out at over 20. Here are some things that make stop allowances lower than normal: lots of dock deliveries; lots of multi-delivery carries (more than one delivery on a single carry); lots of high density driver release residentials...
Here is my experience with time studies, and I've told operations management this many times... About 10% are "tight" and about 10% are "loose". The 80% in the middle are just fine. Check with the swing drivers (cover drivers?) in your center and they will tell you exactly which ones are which. Your only issue is whether a travel variance was improperly applied in one of the baseline units you are delivering. Ask your supervisor to print out a planned day for you on the day you are interested in and see if any of the delivery or PU units are extremely low in hours per stop (below 0.02 for delivery or 0.017 for PU). (As an on-road sup and an IE sup/mgr I never had any issues with reviewing a driver's planned day worksheet with him. If your on-car sup is uncomfortable with this review, ask that your package IE rep conduct the review with you... It's part of their job...) If this is the case, ask him/her to check with the IE department to see if any large negative travel variances had been applied in these units. It is a definite no-no to apply travel variances in baseline units, so this may help.
If you are interested and if your sup is agreeable, ask them to print out a planned day for the area with you and another driver on a different day and compare the calculations. The only differences should be in travel, which is pretty complicated, but in no way related to the driver... Jake