You can interpret each level as “up to N%”. The output is always randomised and then influenced by your settings.
Adverts with a weight of 2 have a change of up to 20% to be shown. 4, up to 40% and so on. Of-course it’s “up-to” because features like Geo Targeting simply deny adverts for some regions. Schedules may expire adverts. You can set impression limits and so on…