"If you step onto the escalator at the bottom and keep holding the
rail, then by the time you reach the top your arm has been pulled
about 18 inches forwards relative to the rest of your body! "

Quick question - do you hang on all the way round the top curve (where
the escalator makes the transition from the incline to the flat - only
to the very top, not to where the handrail disappears back into the
gizzards of the machine*) ?

If the handrail were to travel at the same speed as the stepchain it
would lag behind severely on the top curve, as the radius of the curve
is larger than the radius of the stepchain. Hence it is possible that
the handrail is geared sufficiently so as to never go backwards
relative to the passenger, giving a "creep" on the incline that is
counteracted in the last few metres.

*footnote : the little bit of leather where the handrail did this on
older escalators used to be called an "elephant ear", 'cos that's what
it looked like ...