It may look as the bars are proportional to counts with 100% equal to the max count in the column. However, it's not the case. If you look closely at the screenshot above you can notice that values 02 and 08 have full bars (i.e. 100%) but their counts are not equal (2,236 and 2,922 respectively). More than that -- when no filter applied, all bars are at 100% regardless of the counts. OK, what do they mean then?
Each bar shows the selection ratio for each value and its value is calculated as count for current selection / total count of that particular value.
The best way to understand the bars is to think of them as of an inclusion indicator. They show the degree to which the current selection includes particular value: 0 means current selection doesn't include the value at all, while 100% means that current selection includes all rows with the value. That's why when no filters applied all bars are at 100%. For instance if dataset has field "Gender" with two possible values: "Male" and "Female", the bars would show to what degree the current selection includes males and females present in the dataset.
