Here is my guess based on my experiments due to the same question. I am assuming that the stairs are mechanically linked to the drive motor via chain or gearing for safety and reliability but the handrail is not for whatever reason. I think it is driven using a belt or wheel which is subject to slippage. Next time you are on one have you and 5 other friends pull back on the handrail. You can make it stop and slip backwards. This would perhaps explain it moving slower, the friction between it and the mounts as it moves along could force a little bit of slipping between it and its drive belt or wheel causing it to move slower.

Once again, this is all a guess but I like it anyways.