Why do theist often drop the letter s when referring to atheists?

[FatAndFaithless]

To be fair, atheist is a tricky word to spell if you're not familiar with it.  The e and the i can flip around pretty easily.  In general though, pointing out someone's grammar/spelling is generally an irrelevant, sometimes dickish move, unless it's genuinely hindering the communication of ideas/points.

I’m horrible at spelling, and I don’t think it’s a big deal to correct if someone is constantly misspelling something. But some make it into a bigger deal than that.

Fair, but it's really hard to come off as sincere over the internet when correcting someone's spelling/grammar since it's so often used as a trolling/insulting tool.  I try to highlight it only when I'm genuinely having trouble understanding the structure/content of a statement.