There is only 20bits for the address in the PLB message format - so only 4" resolution.
If you allowed more accuracy you would have to have different formats for different beacons and there would be further confusion when the resulting position string was passed onto the local SAR (is this deg mm ss or deg mm.mm, is this WGS84, NAD83, NAD27CONUS etc. at +/-100m these don't matter)

The standard was developed for marine rescues and 100m accuracy is probably good enough to find you from a helicopter. They don't need to know which side of the life raft you sent the distress call from.

SPOT don't have to conform to any message format other than their own standards so probably send the full accuracy of the GPS chip, 5-10m in ideal condition. How much this helps in a real rescue is debatable - I can't imagine rescuers walking up and down a rescue scene looking only at their own GPS displays looking for an exact spot.