Thermistor reference voltage

[mrbi11]

It appears thermistor reference voltage is OVREF set by zener D12
However, thermistor circuits create resistor divider on VCC

If I read this right, a slight change in vcc will cause temperature to read quite a lot differently?
Same goes for zener value of D12, slight change affects reading?

Is there a reason OVREF is not divided off VCC to mirror thermistor circuit?
In other words to factor out specific voltage values?

Or am I reading this wrong?
Is the actual reference internal to Arduino.
If so, why is OVREF in the thermistor section of schematic?

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edit

Ok, I see ov ref is voltage clamp, not an actual reference.
Please disregard question.

[mrbi11]

Okay, different question.

If ovref is a tad low, won't it appear as of a thermistor is connected when it is not?