The differences in the formula make the differences in the action of the developer : D-94 is a high contrast developer well suited for bw reversal whereas D-76 is a rather low contrast developer, thus not well suited for bw revereal.
In effect, the amount of hydroquinone which is responsible for high densities on the film, is too low in D-76. What's more, there isn't any sodium hydroxide, which as a strong alkaline buffer removes fog strongly and contributes to high contrast. So using D76 you will end up with a grey positive.
Tofek, thank-you. That's a very clear explanation! So I guess D94a has to be mixed from scratch? I'll stick to PQ Universal for the time being. I have ordered some sodium thiosulphate to add to it.