Certainly not an HVAC guy but I have seen that before but usually with older simpler thermostats that could adjust temperature but nothing else. The switch was just used to turn on the fan to circulate the air. Perhaps the heat and a/c went in at different times and they just evolved to newer thermostats using the existing signal wires with fewer conductors. If there are enough conductors or you can fish new ones, you could likely convert one thermostat to do all and abandon the rest. No doubt smarter folks than I will be along.