I also notice that digital tuners are not all exactly the same. My old Seiko was slightly different than the Boss tuners. And perhaps the pitch reading dropped when the battery was weak. Phone apps could need calibrating as well.
Stuff in the digital domain is usually pretty accurate. That said, the algorithm for a tuner can be quite complex - there's all sorts of averaging going on, so the fact that different tuners show different results is more likely to be a result of the different way each has decided to turn their measurements into feedback they show you, rather than inaccuracies of their measurements, IMO.
The Peterson strobe tuners, which you can now get in clip-on versions, are the gold standard as far as I'm concerned - after I tried them, I couldn't make myself use anything else. Expensive, $50 apiece or so for something you can buy for much less, but they're great and worth the $$ to me.
If you want to try something cool with metronomes, not tuners, take a couple of smart phones running a metronome app and put one at quarters and the other as eighths, e.g., set one to 72 and the other to 144. If you start the second metronome app exactly in sync with the first - it might take a few tries but it's perfectly doable - you'll find they stay exactly in sync with each other for at least as long as I've ever tried it, maybe 10 minutes, and I suspect they'd go on this way forever. It's way cool, IMO, and certainly not something you could ever do with the windup kind of metronome. (I use this with students all the time because I can adjust the volume of each metronome, sometimes by just moving it closer or further away - great for someone who can't, e.g., get 8ths in tempo at 72 to hear where the 8th are, then gradually reduce the volume of the 8th.)
-S-