xkcd and Braille

xkcd had a comic featuring Braille:

Now I'm wondering why "ed" is contracted to dots-1246 but "gh" is not contracted to dots-126 in the word "sighted". I also wonder whether "people" doesn't have a contraction... *looks up* ah, "p" by itself.

Seeing an uncontracted "people" didn't bother me so much, since I imagine Braille on public signs tends to be relatively uncontracted since not every blind person will know Grade 2 Braille. But I wonder, then why "ed" made it in? And if "ed" is in, why not "gh"? Both signs would seem to me to be part of a reasonably "Grade 1.2 Braille" (along with other letter combinations such as "th", "ing", and "sh"), given that they have (nearly) no other meanings.