My brother and I went on a walk together to try and hatch eggs we'd been working on. However, at the end of the walk, he had registered significantly more distance towards hatching his egg than I had. We both had the app open and our phones unlocked for pretty much the entire walk, but my brother did seem to be having better luck with data reception/getting the servers to respond than I was.
I theorized that because my phone was not communicating as readily with the game servers, stretches of the walk where my phone failed to report distance traveled to the game servers also didn't count towards hatching my egg. Is there evidence that distance traveled has to be reported to the server to count towards hatching an egg (as opposed to distance towards hatching an egg being tracked primarily by my phone, without server involvement)?