It is probably because it is set to double portaging. When you double portage, you walk the portage 3 times. The Route Builder takes this into account, so it looks like the portages are three times longer than what you expect. You can change it to single portaging.

If you are just looking at portage lengths in the Explore mode, these are calculated. If I have a GPX track, I use that. If not, I have done my best to estimate where the portage is.