As it turns out, roots are considered "hot hair." According to Jeanise, this is because body heat accelerates the process of bleach working. For that reason, the first round of bleach is applied only to "cold hair," since that takes more time to oxidise (that's the big-girl word for stripping away the hair's original colour).
In this picture, you can see how Jeanise is leaving my "hot hair" be . . . for now.