It is common with smaller sized Relief Valves. Typically these are of the Poppet style, although Seat Disk are common. Because of thread loading and associated stresses in the make-up, the smaller sizes allow low pressure class and stagnation pressure when the valve is fully seated.
I'm not sure where the cut off is with valve size. You should be able to distinquish this easily in a Parker-Hannifin catalog or equivalent manufacturer.
Regards,
Cockroach