You are right. There is something strange in that area.
I found a website that can be translated about this area.
https://ferlir.is/nupshlidarhals-2/

Seems like this area should be considered as a rift and has multiple peaks.

However, there is this website visiticeland.com/plan-your-trip/travel-search/search-results/view/reykjanes-nature-reserve showing it's around 300m, but I doubt it. We need something more accurate.