A +1 to hit turns one more face of the d20 into a hit. That face is 5% of the die, but it is not 5% more hits. When only five faces hit, the sixth is one more for every five (20% more hits). The harder the target, the more every +1 is worth.
On the chart, your roll is the outlined bar (or the dashed line). The harder the target, the fewer faces hit, so each extra face counts for more and the bars climb.
Top row as you are, bottom row with the extra bonus. The outlined faces are the only ones it changes.
The odds are exact, worked out from every face of the d20, with the same engine as the Strike Lab.