Maybe it's all the sleep he needs. Some LOs this age do 3hr or 3hr 15 A times, others do much much longer. So if you are putting him down at around 3hr 15 for each nap and again 3hr A time before BT then it makes sense that there is less time for day time sleep. If the second nap was longer (and might need a longer A time before it) then BT could end up much later. An 11.5hr to 12hr night is pretty good. LOs who can do these longer nights will often end up with one of the naps being shorter. LOs who have longer naps often end up with the night being shorter - I'm not sure you'd be happy with a night of only 10.5hrs when you know your LO can do closer to 11.5/12hr.
You could try an additional 15 min on that A time before nap 2 and see if it's any easier to resettle or do a W2S but you need to be prepared then for the chance of BT moving even later and the night being shorter.
hope this helps