I don’t know if any commentator ever attempted to show how the figure might be arrived at, but if they did I expect it will have been done using the usual method by which Ābhidhammikas arrive at extraordinarily large numbers. For example:

The Bhikkhupātimokkha comprises 227 *sikkhāpadas*.

Multiply by 3 according to whether the bhikkhu is a *puthujjana*, *sekha* or *asekha* = 681.

Multiply by 2 according to whether the citta that caused a *sikkhāpada* to be observed was *saṅkhārika* or *asaṅkhārika* = 1,362.

Multiply by 2 according to whether the citta was *ñāṇasampayutta* or *ñāṇavippayutta* = 2,724.

Multiply by the three periods of time, past, present and future = 8,172.

Multiply by the two regions, Majjhimapadesa and Paccantapadesa = 16,344

Etc., etc.