Are you referring to the fact that if the region is dark green (more than 20%) for moving into the top quintile from the bottom, it necessitates that another quintile has a rate less than 20%?

The income quintile boundaries are set nationally, but the movements are divided locally, there is no such requirement for balancing in a single area. In theory, a single location could have 100% of people moving into the national top quintile.

edit: I'm referring to the chart called Rags-to-riches income mobility