Looking on some wheel manufacturers websites, they just mention the rim width as X amount of mm…. 23mm, 28mm… whatever. On the Mavic site they state "internal width". Is this how others state rim width?

Like you've already experienced, it depends on the rim manufacturer.

Many measure externally, Mavic to be different measure internally.

FWIW, the Mavic rims I've had and measured, you can add 6-7mm to the internal quoted width to work out the external width. ie.

717's are 17mm internal, 23mm external
719's are 19mm internal, 25mm external
721's are 21mm internal, 28mm external
823's are 23mm internal, 30mm external

etc.

If you do some digging you might find them somewhere on the new. MAvics are easy as I'm sure they have drawings on the relevant pages of the website. So to do stan's on the notubes website. I guess DT, and the others are the same.