Exactly. Perfect example.

They are their own power supplies.
If VDD to the PIC changes, the other voltages don't necessarily change.
So there is no Ratiometric relationship, and a Fixed reference is required (like you have).

But when reading POTs, many pressure transducers, or any sensor that is resistive in nature, it will likely be ratiometric and a Fixed reference will distort the readings.

I guess I just thought that this was a bit limiting.
They try to get 10-12 bits (or more) of resolution when using Vcc as the reference voltage. That just doesn't work.
It's quite possible to get 12-bits or more without a Fixed reference.
If you are reading a Ratiometric device.

Fortunately, whether or not a reference is used can be changed in the ADCON1 register, so even within the same project you might have some Fixed or unreferenced sources, and other ratiometric devices that change with VDD.

If you know the difference, Use the reference for the fixed sources, and no reference (VDD) for the ratiometric ones.