The reason behind the regulation is simply to distinguish between the 25kHz frequency xxx,350/or the 8.33kHz channel xxx,355 or for example ,850/,855. In those cases both selections would most propable put you through readable to the right controller.
Problem is:
If you select ,850 (25kHz) instead of ,855 (8.33kHz) you will most propable also cause/recieve interference on the ,835 and ,860 which uses the same carrier. And of course they are propably allocated within recieving range otherwise the switch to 8.33 wouldn´t have had any benefit in that area for those freqs...