Both that one and the gym one are obvious "exceptions with a rough analogue" - the nunnery is allowed less parking than would normally be required because of the assumption of communal living (the same reason one vehicle per household makes more sense than one vehicle per driver when dealing with residential units).

And gyms are unlikely to make deeper pools, but longer/wider ones, which would support more customers.

And when the regulations were written down, you'd only need one nunnery in development or existing or foreseen to make the exception appear.