The thrill of weaving through traffic vs the tedium of being the traffic might be the real incentive, whether the driver is consciously aware of that or not.

I just drive normal and stick to the right unless I'm passing and try to maintain a good speed the whole time - no breaking and reaccelerating. I often see the people weaving and then pass them 5 minutes later because they tried to pass on the right and got stuck behind a semi. Ha.

Is it surprising though?

Not really, it can save serious time if done properly and carefully in a consistent way on a long distance drive. Driving the length of California on I5, you can easily get stuck behind side-by-side slow traffic and spend the entire drive averaging about 60-65mph. Or you can aggressively cut through this isolated 'island' of slowness, and average 80-85mph. Over the ~400 miles from SF area to SoCal, this saves about 1.5 hours each way.

I used to have to do this round trip commute, and could consistently save ~3 hours per week of time by driving more aggressively, and I never got a ticket driving like this doing the commute weekly for years.

I do however try to be as courteous and safe as possible, and would time my lane changes to maintain safe following distances and not actually cut people off. If people would stay right except while passing like they're supposed to, this wouldn't be needed.