Notice that the shaded levels come in from the left side, while taxes cut in from the right? The shaded levels denote the proportion of each tier (starting on the left side) which would be required to be left untouched by taxes (which roll in from the right) in order to give the average family in that tier the "take-home" income stated (22350/44700/etc). If the taxes take too big of a bite out of that right side (ie, if they overlap the designated income border), then that means there is not enough TOTAL money in that quantile to theoretically give every family in that quantile the income stated (if split equally among all families in that quantile, that is).
What he's basically saying is that the bottom 20% earns an amount of money amongst all of them such that, if pooled together, there would be enough for every family to have a poverty-line income ($22,350/yr) after taxes, but there is not enough total money for everyone in the bottom 20% to have the $44700/yr which is his next graduation. If his scale is to be believed (that is, if his little yellow money blocks are accurate), then the total amount of the bottom quantile, if split evenly, would give each family (that's each family, not each person) about $28,100/yr after taxes (if we eliminate the taxes portion, it looks like there's about 13.6 columns of yellow blocks out of 17.1 total columns, so 17.1/13.6*22,350=28,100). So basically the bottom 20% are just completely hosed if they want to live any sort of a meaningful lifestyle.
--Patrick