Pure speculation...

Planned redispatching? To reduce reserve fuel requirements they plan to Guam, say, then to NRT. Normally when abeam of GUam on the way to NRT, they re-evaluate fuel and dispatch to NRT. If they can't meet requirements to redispatch, they have to go to Guam. Perhaps they burned a lot more than expected on the start, headed to Guam, then learned they had enough to keep going??? I have no idea if Delta follows this practice.