I have often wondered about the activation threshold for the low RRPM horn. The governor controls at 104% but it presumably has to see a small error before it responds to it, hence the actual RPM must vary within a dead band. In a previous life I was a control systems engineer, and I don't think the governor on an R22 is particularly sophisticated. I assume that if the horn warning level was set much higher, the risk of spurious warnings under normal operation would be higher. If the thing went off too often, then the user would get accustomed to it and maybe ignore it on the one time it really did matter.
During R22 PPL training, you do range autos at 90% rrpm if I remember correctly. Hence 97% has quite a large safety factor as a warning setting. I guess it depends on the rrpm decay rate which itself depends on the power demand and pitch setting.
Presumably like most things it has been chosen as an acceptable compromise.