Probably because the two frequencies are so close together. But, the receiver may not be quite as sensitive on the other frequency as it is on the one it's tuned for which will effect the receiver's overall range. However, if you tried putting a 72 MHz crystal in a 35 or 36 MHz receiver, it probably wouldn't work at all even considering that all other things were equal (AM vs FM, single conversion vs dual conversion, etc.).

In the US, our FM commercial radios cover from 88 MHz to 108 MHz using a single receiver.
I realize these radios are designed to cover that range, but they will also receive outside those frequencies, although to a lesser degree. It's the mechanical or electrical programming that prevents the receivers from going outside their designed frequency range, not the capacity of the receiver's tuned circuits.