It's probably a result of improved exhaust scavenging due to changes in the transfer ports. Generally when the scavenging is poor the percentage of residual (burned) exhaust is high in the combustion chamber so additional fuel is needed to get a mixture that will consistently burn. Improving the scavenging makes it easier for the oxygen molecules to find a fuel partner so less fuel is needed.
So would that mean that on top end the cylinder scavaging is improved and making more power?
If so why does everyone claim that a rad valve is'nt any good for top end becuase when you install one you also have to go leaner on the main.
The reason for my curiosity is that on my 86 CR500 I had Erric Gore port work and a rad valve, bored carb, K&N air filter and PC pipe and I went from a 172 main to a 162 and probably could get by with a 160 main.
Old exhaust products just cool the combustion chamber and take up space, so the lower the perscentage that get left behind the greater the potential power. A reed valve's effect on jetting is caused by a different set of conditions within the engine.