Where did you get that image from?
What's weird in that image is that according to it, the F-117 is "stealthier" (have less RCS) than the B-2. That's weird because every information that I read so far indicates exactly the opposite: that the B-2 has in fact a lower RCS than the F-117. I also read that the B-2 is currently the stealthier aircraft in existence (at least regarding full production in service combat aircraft).
Resuming, what I read regarding US Stealth combat aircraft RCS is as follows:
- The B-2 is the most stealth of all US Stealth combat aircraft (the one with the lower RCS)
- Next (in "2nd place") comes the F-22 and the F-117 which have a very similar (or almost identical) RCS
- Finally (in 3rd) comes the F-35.
Albeit that, I also read that the F-35 is stealthier than the F-117 regarding frontal arc but in the remaining arcs (side and rear) and in overall/average the F-117 is stealthier than the F-35. This of course means that an aircraft RCS will differ depending on the angle/arc which is facing the radar.
Finally I read that the RCS value that we usually see for a particular aircraft (like the ones seen in that picture) are usually an average value from every RCS from the various different aircraft arc/angles.
This information was mainly gathered from a very interesting article that came in a Combat Aircraft Magazine issue, regarding the history of stealth and it's application on US stealth aircraft (F-117, B-2, F-22 and F-35), this among other sources.