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Not having the schematic infront of me the most likely reason would be that the cathode is at a higher voltage relative to the main heater winding and was given a separate winding so it's heater could float to that potential and avoid heater cathode shorts.
If you want to talk weird heater wiring I have a Bell/TRW tube stereo reciever where the phono/preamp stage 12AX7 heaters are used as the cathode resistor for the audio output stages thus saving parts and driving those heaters with DC. There were IIRC GE sets in the late 40s that drove one half of a center tapped tuner tube heater with DC while the other half got AC...And let's not forget Zenith pulse regulator color sets connecting the Horizontal output and HV reg heaters in series on their own heater winding so the other tube would not work with one removed...That also caused certain compatible tube substitutions to have to be done in pairs.
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