Apparently you're significant enough to warrant the Pemco ad I see on buses all over town. But what the hell does "I know I am I'm sure I am" mean?

I can only imagine it's a chant favored by die-hard fans (who both know and are sure of something), but please confirm. To quote Lisa Simpson, I know those words, but that sign doesn't make any sense.

Also, is what I'm feeling now what straight guys feel when some gay guy yells "Butcha ARE in that wheelchair, Blanche!" in their faces?

(Also also, both "busses" and "buses" look wrong. Get on that, science.)