I've always heard the reason given for the velocity increase as freebore boost.
To quote Robbie Tiffen (Canterbury Gunsmith and Suppressor Manufacturer)
There is also the velocity phenomenon known as "freebore boost". To explain: When a fired projectile uncorks from a rifle bore, the gas expands and dissipates into the atmosphere, causing the blast that we are familiar with. When a suppressor is fitted, the projectile uncorks inside the body of the suppressor and gas continues to work against the base of the projectile, increasing velocity slightly until the projectile exits the suppressor front cap.
It is New Zealand's role to send out its bright young men and women to help run the rest of the world. And they go, not hating the country of their birth, but loving it. From this loving base they make their mark on the world.
Pro-1080 Poison and proud of it!!