I would say that's the incubator changing temp. Even in my Brinsea, I measure temps from 98.9-100.0. It's just the nature of thermal flow. The spot check reads quickly once it comes up to incubator temperature, so it will rise and fall with every different spot in regards to its distance from...
I will say what I do with the spot check, and take it for what it's worth. Since the wire for the probe is somewhat malleable, with a little manipulation I can "drive" it anywhere in the incubator. I took as many readings as I could from the center, corners, and random spots. I took an average...