tell me one, which is linear, means, is not demodulating in the GHz range.
An instrumentation amplifier, just the very high amplifying ones have a rather poor bandwidth. They can never handle cellphone radiation frequency. So, at this high frequency 'TRUE differential input' does not exist.
This is where you employ a lowpass filter. A lowpass filter for some reason, that one of the more scientifically inclined may be able to answer, works as a charm on a differential input and not very well on a single ended input.

Erik