My dd is on one nap and does 5 hr of A time in the am, but only 3.5 after nap/before bedtime - which I find odd too, but it works, so I'm running with that!
I also noticed when she was going through the switch (started around 11 mos, mostly done by 14 mos), we'd have both 1 nap days and 2 nap days. In the early stages, I found for every 1 nap day, she needed 2 -3 2 nap days to not get chronically OT, then gradually, it was only 2 days to recover, then we alternated every other day, etc. until it was more 1 nap days than 2 nap days and finally, no 2 nap days at all. If I pushed the 1 nap days, the chronic OT set in and I was doomed.