Pumkin? Really? One 'P'?

In one of my classes, I use a domestically produced textbook. It is published by a famous hagwon in Korea and it isn't Min Byoung Cheol. Anyway, it is full of spelling errors. If the book is to be trusted, fashionable women wear 'high hill' shoes and students can correct errors with an 'erazer'.

I thought this was another error but now I wonder if it is another American/Canadian/Other spelling mistake (Sorry Australia, England, New Zealand, for lumping you together like that). Is the big orange gourd, a symbol for Hallowe'en, spelled 'pumkin' or 'pumpkin'? Google finds websites with either spelling. The Korea Herald has this photo with the caption 'Pumkins for display only'.

The caption below (my coloring):

A Lotte Department Store displays a variety of pumpkins, a popular ingredient of "Juk," light porridge.

The Herald is giving both spellings in one article. Perhaps the editors were not sure and decided to be wrong once rather than twice.