[jcx]

basic listening test standards are harder to apply/verify with electrostat amp/headphones - the fast, clue free Blind switching between amps, verifying level matching require specialized parts - much rarer, more expensive, more ways to fail than with  lower V, lower Z dynamic headphones
 
calibrating level with microphone and head/ear simulator are possibly easier but more variable than measuring the headphone drive V of electrostats - see Tyll's InnerFidelity headphone testing setup, reproducibility articles
 
then you have the expense of the amps - only a few ever have several models together at the same time so many posted impressions are relying on our extremely faulty, malleable "sound memory" which nearly always compounded by the lack of level matching

 

[castleofargh]

well it could also be that they make one amp for one headphone and try to finalize the signature they couldn't "perfectly" set in the headphone, with coloring the amp? 
but I really don't know if that's even really a thing manufacturers do.

 

[stv014]

Electrostatic headphones are more challenging to drive than others, due to the large voltage swing (even compared to loudspeakers) that is required in the range of up to hundreds of Volts, and they are a very small market, so it is not impossible that some of the few amplifiers that are available are not transparent. But it is hard to tell without detailed measurements, and sighted listening is not a reliable source of information.

 

[blades]

   
And I am referring to amps that can drive the headphones just fine, yet sound different.

 
Two possibilities.  The most obvious is that the comparisons were not made with level matched, bias controlled comparisons making the results invalid and meaningless.  The less obvious possibility would be a severe impedance mismatch between the amp and the headphone.