Being a long time user of 27, I have never had the problem. I tend never to bother about the actual frequency, just the colour coding of the crystal.
It is possible that one of the sets is duff. This could be either a broken set, or an RTR with no crystal changing facility, which, from a satisfaction point of view, seem to be designed that way. These are often designed for low cost, and typically have low power output and poor sensitivity along with poor selectivity. As a result, they tend to be swamped by anything else anywhere near their band.
Over the years, the number of allocated channels within the band has increased - older equipment can still be used, but there may be some issues when mixing new and old. An old transmitter might well be producing signal in neighbouring channels, and a receiver might well try to resolve information being broadcast on them. This may also happen if the sets lack either an FCC (US approval) or CE (Euro approval) label. This indicates that they where made without troubling the certification authorities, and thus might not conform to any known standard. Interference is a symptom of this.