Note only a small percentage of the hydrogen escape specifically the ones with the higher velocities. The leftover distribution after these particle escape is no longer a boltzmann distribution and is therefore not in thermal equilibrium. It will change gradually to try to become a boltzmann distribution again. This then allows more particle to be faster and this cycle acts as a slow leak over time I believe at an exponentially decaying speed. This is likely what the bands are trying to show i.e. that a planet just cold may still have these gasses for a decent amount of time.
This is called the jeans escape process.