240V wiring and a thermostat
With the limited info provided I will guess that it is a 240v heater as the other one was but this time they only used a single pole thermostat to control the power, it works the negative here is that there will always be a hot conductor in the baseboard unless the breaker is turned off, this used to be quite common, or was used with multiple heaters in 1 room. Each of the switches in the thermostat can control a 240v load, but by not breaking both hot conductors there is always power to the heater but no current flow until the switch closes providing a path for current flow.