I guess most of them don’t work on statistics for a living though?I'm not aware of any audio professional, or anyone else for that matter, who claims to be able to measure bass as a percentage.
I mean there is probably no clear standard on that, but if the old monitor goes down to 70hz and the new one goes to 60hz and you consider the limit of hearing to be around 20hz then the old one would have left out 50hz while the new one would leave out 40, a difference of 10hz.
Counting that into a percentage would be simple, though it would still be pure marketing. Generally those are specified based on the sound level dropping by some amount of decibels and again there is no standard for that (better speakers use a smaller drop).
At the end of the day, I guess what they are saying is just that it goes a bit deeper.