HDX amazon vs Kobo charger

[cybmole]

I own 2 chargers
the amazon one that came with HDX ( a.k.a. the accelerated one that sells for silly money (with mixed ratings) on their web site)
and the Kobo arc charger that came with matching tablet.

Amazon's charger is rated at 1.8 amp
but Kobo is rated at 2 amp ( on it's label)

So how come when I connect the KF to the kobo charger is warns me that I'm using a low power charger & that charging will be slow?

Is the KF really measuring & commenting on incoming current, or is it looking for some proprietary pin signal that only amazon's branded device delivers. Or are Kobo exaggerating their rating ?. I don't have any electricians test equipment.
:d like to separate the science from any branding or labelling mischievousness.

Surely the charge is always delivered vis the same pins, otherwise the chargers would not be interchangeable, so either someone is playing fast & loose with power ratings, or amazon are using some extra proprietary trickery?