Yeah, it could be that the gauge is off or...but for now, let's assume it means what it says.
I do have a small, but persistent leak from my NOS Alfetta A/C & GTV6 type heater water valve. I'll add that it it is covered in JB Weld, just as my other NOS valve is and they still insist on leaking.
I have an on/off valve in the engine bay and it is currently turned off, and a hose clamp - as in clamping the hose closed - on the return line. Meaning the water isn't circulating through the heater. But I've always had the interior heater closed for most of the year.
Perhaps the leak brings down the pressure, but why wouldn't the gauge indicate a higher temperature?
It almost seems as if the head isn't getting enough water flow, but that's just speculation.