UL 1971: is there a tolerance on strobe flash rates?
Don’t have easy access to that standard but NFPA 72 is supposed to incorporate the UL standard and it says:

18.5.3 Light, Color, and Pulse Characteristics.
18.5.3.1 The flash rate shall not exceed two flashes per sec- ond (2 Hz) nor be less than one flash every second (1 Hz) throughout the
listed voltage range of the appliance.

So no tolerance is, strictly speaking, allowed for, so I suppose a prudent designer should aim to be within the tolerance range over temperature and voltage and all other variations. Of course if the nominal rate is 1.01 or 1.001Hz, the data sheet for the alarm will likely state 1Hz, so without measuring the actual rate it is not possible to tell what they have done.