One situation where you could see a big gain in flow as the lift starts to get past .500” is if the valve is sort of “way too big” for the chamber, and the chamber wall is quite close in proximity to the valve for a large portion of the circumference of the valve.
Then when you finally get a large enough curtain area exposed thru high lifts, the port takes off.
That being said, a 55cfm jump from .500-.600 out of a W5 head still seems like a stretch to me.