Consider the 'base' terminology as being but the carrier that has to sufficiently emulsify/disolve the resins, oils, pigments, etc. to get it to the wood. It evaporates leaving such and such of a solid content/thickness. Now consider alkyd emulsions a mix of both water and oil as base/carrier. They don't hold or effectively disolve as much resin,oils,pigments, etc. and so the solid content is dropped. End product should be able to be made same end quality but most agree they just aint there yet due to content sold for one thing.
In your synthetic acrylics things of metal crosslinking (makes resistant to chemical attack) and curing hardness come to play and apparently is hard thing to tackle for exterior products.
Thing that gets me with the VOC stuff is that having to put more of the product on in two coats to get about same thickness is excepted. To me it a hassle of a game in most respects. Like think about it..let's have to make them use more, buy more, and spend more time and labor to get to same place is embrased?..
I'll take catalyzed urethane that is uv stabalized just like on a car..lol