i expect if you are filming for a youtube walkthrough have already played it and dont record your deaths experiments tourism or stopping to try something out for the hell of it, it will be half the length of an average playthrough. Just the nature of it.
of course if you want to be critical you will focus on the lower number and if you admire / enjoy the experience, you will focus on a higher number.
in a game with cinematic style and story driven gameplay, the lower, faster time, is even easier to complain about, if you are looking for reasons not to buy it.