I've noticed this as well. The upshot is that seeing see a reported 100% self powered day via the app is either non existent or very rare. The app can (and routinely does) show a 99% self powered day when the grid data shows 0.0 imported. It also consistently shows what you noticed, if you draw some grid and then exporting more still shows up as less than 100% self powered.
Perhaps there is a justification (as some suggest) for this in definition, but I consider it a flaw. Being a bit of a data hound, I keep my own spreadsheet that reports self powered percentages more accurately. I'm coming up on 10 months of operation, so it remains to be seen if I continue to keep daily data, but for now I'm keeping at it.