Upside-Down Bingo

No, that isn’t a euphemism for the Texas Triangle, the Humpty-Bumpty, or the Hunka-Chunka.
Working a bingo on Saturday, I saw something I didn’t expect to see. A woman playing bingo with the cards upside down.
Rather than feeling ridicule, I was immediately stricken by an amazingly strong curiosity.
Why does this woman play bingo upside-down?
I came [...]