FD - it comes from a set of rules established many many years ago by the authorities, now superseded by EASA. There is an allocated number of passengers per door type (for example, a type III overwing is allocated 35 passengers - I don't have the figures to hand for a Type I but could probably look them up).
Then the authorities dictate that, in an emergency, only half the exits could be in use. So, for, let's say an A319 which has 1 x type III on each side, the max is still 35 pax allocated to those type IIIs. An A320 has 2 x type III on each side, so it has 70 pax allocated to those 2 exits.
So the A321 is limited by its door configuration in the same way as most other jetliners, so that it meets the emergency evacuation requirements. The A321 Standard Specification from Airbus states that all doors on the A321 are Type I, but if I remember correctly the doors immediately forward and aft of the wing are a slightly different dimension to the main pax doors at the very front and very aft, so probably have slightly different limits. In any case, when the A321 was certificated and the emergency evac test was done, only 220 pax were on board so this figure may never be exceeded.
Flyblue is correct in her statement that crew of any kind don't count, but ALL pax do. That is the certification requirement.
Am I helping or making it worse?!
p-k-b