It’s a second (or third) axle that can be lifted so that the wheels do not touch the ground when running light. This has an effect on fuel consumption and tyre wear, and also in some places it can affect the amount paid on tollways.
Does that increase or decrease the toll? It seems as though your vehicle would cause less wear to the road if its weight were spread over an extra set of wheels. On the other hand, the government may figure that only very heavy vehicles would have that many wheels, and thus go by the number of wheels (in contact with the roadway) alone.
Y’know, not being a trucker, i’ve wondered the same thing, applying the same reasoning you have - I’d wager the higher subsequent axle load would actually damage the road more. However, in my jurisdiction at least (about as far from California as you can get), heavy vehicles pay increased tolls as the number of axles goes up. Either the folks in charge of this are spectacularly thick, or more likely this is the simplest way of doing it. California axles are not that common here (not rare either though), so for the most part the toll operators get their appropriate toll from the truckers. The money lost by those who ‘cheat’ by lifting axles is probably less than the money it would cost to install scales or some other system at the toll gates.
I think, from memory, that once you get up towards semi-trailer level, you’ve hit the maximum toll regardless of whether you’re an 18 wheeler or a 22 wheeler etc. I agree it does seem silly and counterintuitive though.
{Here endeth the hijack - apologies}