What either of the two above posts said could be the problem.

Maybe your guitar is out of tune enough that the sixth string is closer to D than E so that is what is being displayed. Did you just put new strings on? Even if you didn't, your 6th string could still be closer to D than E. So go to the tuner app here on GT and get your strings as close as possible audibly. Or use Guitar Tuna or some other app. Get your strings close and then see if it displaying the correct street for the one you are tuning.

And, yes, make sure the Snark is in the right mode for guitar.