I think I understand what you're asking...
The viscosity numbers indicate the relative viscosity of oil when cold (first number) and when hot (second number). In order to make conventional motor oil behave like 50W when it's hot, they have to start with 20W base stock. It's a long story, but I'll try to give you the "Readers Digest" version...
GM spent a ton of money in the eighties on a study of sludge accumulation that they (and several other manufacturers) were having a problem with. They found that the biggest problem was with 10W40 oil. It just took too much V-1 friction modifyer to bring the 10W oil up to 40W specs when hot. The excess V-1 would vaporize and condense on internal engine parts. The result was that all of the dealers (MB included)in warm climates began to use 15W40. Some manufacturers (like Volvo for example) even went so far as to state that they would not honor engine warranty claims if 10W40 was used. 20W50 oil didn't have the same problem since the "base stock" oil was twice as thick as 10W40 (it takes less V-1 to bring a 20W oil up thirty points than a 10W oil).
So anyway, the point is that they don't have to make a 20W50 synthetic since they can start with a lower weight synthetic and still make it behave like it's much higher weight oil when hot. The best of both worlds; oil that flows well when cold, and protects well when hot.
Is science great or what?