I know exactly what you are talking about,
because everything used to be by dry weight all the time.
It seems to no longer be the case, and everything is by simple volume,
and the measurements can be very approximate!
I have mixed up batches that were very different from others,
and everything grew fine. I have even done experimental batches with some
"uncommon" ratios & ingredients, and again, everything works out fine.
A lot of how you base your ratios will need to take your watering habits into consideration.
Less watering needs more peat (or other water holding ingredient), more frequent watering
can use more drainage and hence things like sand, which holds less & drops its moisture faster.
Use common sense and it will all work out fine.
(Seems no one owns a scale any more or is too lazy to weigh things, so volume methods have
replaced the weighing ones. In this case, it doesn't much matter, as there is a lot of flexibility
as far as what works.)
Take care!
[*As you can perhaps tell, the complexities of actual measuring can be a bit "confusing", hence why the simple volume measuring likely was adopted! I simply think of a pound of feathers and a pound of lead! Perhaps keep this experience in mind when requesting future info and opinions! Amazing! But then now-a-days most people today can't even count back change!]