Why is the PME fault detection threshold for single-phase charging piles in the UK < 207VAC or > 253VAC?

It is quite simply because the EV charging equipment standard, BS EN 61851-1:2011 stated in Clause 5 (see my highlight):

5 Rating of the supply a.c. voltage

The rated value of the a.c. supplied voltage for the charging equipment is up to 1 000 V. The equipment shall operate correctly within ±10 % of the standard nominal voltage. The rated value of the frequency is 50 Hz ± 1 % or 60 Hz ± 1 %.

NOTE Nominal voltage values can be found in IEC 60038.

I am aware that this is not the current version of the charging equipment standard standard.

There are a number of reasons a wider voltage range would be preferable, but this has to be balanced with safety.

The IET is currently working on a new standard specifically for open-PEN detection devices, that will hopefully be available in the Autumn, which proposed this range is increase (but other measures may be required for safety).

It is quite simply because the EV charging equipment standard, BS EN 61851-1:2011 stated in Clause 5 (see my highlight):

5 Rating of the supply a.c. voltage

The rated value of the a.c. supplied voltage for the charging equipment is up to 1 000 V. The equipment shall operate correctly within ±10 % of the standard nominal voltage. The rated value of the frequency is 50 Hz ± 1 % or 60 Hz ± 1 %.

NOTE Nominal voltage values can be found in IEC 60038.

I am aware that this is not the current version of the charging equipment standard standard.

There are a number of reasons a wider voltage range would be preferable, but this has to be balanced with safety.

The IET is currently working on a new standard specifically for open-PEN detection devices, that will hopefully be available in the Autumn, which proposed this range is increase (but other measures may be required for safety).