Which temperature to use?

Which temperature should I use? This temp of 98.05 was at 6:15am upon just waking. Then I checked my temp maybe 10 mins later, and it had dropped to 97.41. I know everyone says to go with the very first temp upon waking, and at the same time every day. I always take my temps between 6:00-6:20am. I'm confused as to why it would have dropped so much in 10 mins after being awake? I linked two charts showing the huge difference it makes with how each one would be logged.