The head tube length and seat tube length (5.9") match up with a 20" frame, but that should mean a 24" effective top tube for this Hei Hei....
Maybe its worth checking the effective measurement again. In the past I've drawn out the geometric centrelines on a desk to confirm the effective top tube length of some frames. If you work with a 71deg head angle and 73deg seat angle from a reference hoziontal line you should be able to confirm the eff top tube without doubt. It may just be that the frame was made with a shorter top tube for someone(?)...
In the end it probably won't matter much as the frame is obviously going to suit taller person.
Anyway...
I take it the stickers are new from previous comments about the powder blue ones underneath. Are you also able to tell me the frame number?
Cheers
Andrew