yeh, scale is subjective.
Redcat has 2 1/10 scale offroad trucks...the volcano and the caldera. But the caldera is a couple inches longer and wider as well.
The hpi wheely king is 1/12 scale, but nearly the size of most 1/10 scale trucks.
Short course trucks that are 1/10 scale are much longer and a bit wider than most 1/10 buggies and stadium trucks.
some models that are based on a real-world vehicle may be legitimately "X" scale of the real thing. But most monster trucks, buggies, stadium truck ect are not based on any real vehicle.
Redcat also has 1/5 gas powered trucks...but these are a fair bit smaller than the Losi 1/5 short course truck.
So scale is just a loose number to go by.