I thought that the Aldgate bit only ran at rush hours and whilst I can see
that a last train at about 20:00 is reasonably still inside the "travel home
from the city" work hours, I can't believe that it has a two minute
frequency at that time of day.

Met trains to Uxbridge run to Aldgate at any time, not only during
peaks.
During peak hours extra trains from Amersham and Watford are added to
Aldgate service.

Two minute frequency was either because of previous overcrowding or in
response to Circle line usual "lack of drivers".