Being a community of American educators, we've gotten a number of comments about this! :)
The software that we use for our online course was predominantly designed by developers in Australia, where - like in the UK and New Zealand - "enrol" is an acceptable variant spelling of "enroll." However, because we are at the moment a community of American educators, we are working on changing the spelling throughout the site to the variation that's more standard in the US.
We apologize for any confusion this may have caused - if it makes you feel any better, we didn't know "enrol" was a thing before our pilot program, either!