It could be what sweetcocoa suggested and it could also be, that the relaxer doesn't remain in that area as long as the rest of the hair, especially if it is touched up last. A lot of people find the hairline one of the coarsest areas, but also because of the delicacy of this area it left till last or has a shorter amount of time where the cream is left on.
When I relaxed Nicole's hair for the first time a few weeks ago, we encountered the same thing. I told her not to worry and that I didn't want to chance leaving the cream on in that area because
1. It was my first time doing a touch up on someone
2. It was her first time using the Affirm Relaxer.
It was slightly rougher compared to the rest of the hair, but it's because it was on only for a few minutes, it was very coarse and it was left till last.
If your hair in that area isn't stubbly and short and looks broken off then it's possible that's what could have happened.