The dram system was based on the number of drams (apothecary scale) of black powder used to propel a given shot charge to a given velocity.
When smokeless came out, the early type was bulk which was substituted one-for-one volume with the BP, no problemo. when they got a little more sophisticated and came out with double base (I think) powder, much less was needed to get the required velocity. That's where they had to come up with the dram equiv rating as everybody in those days was familiar with the dram scale.
Today, that is so far in the past that MAX is as good as any way to describe a load. I've also seen shotshells marked with velocity ratings rather than dram equivalents, IIRC.