[Pat Fairbank]

I would imagine the speed limiting function further complicates the balancing algorithm.

When you're approaching the maximum speed for the key you're on, or when you're otherwise approaching the maximum power the motors can put out (like when facing a strong headwind) the motors sort of put on a burst of acceleration to get the base of the Segway further forward in relation to the handlebars, so that if you wanted to accelerate any more you'd have to lean right over the handlebars (which would prompt a stick-shake warning).

So I'm going to leave speculation on how it works to people who are more qualified to answer.