I just compared the two maps (Navionics and EPA) more carefully, and they don't actually agree like I thought they did. The EPA map is much closer to the actual depth, in the spot in question. I'm talking about the southeast part of the lake. Navionics makes it look like a steeper dropoff than it is.
Navionics wasn't as wonky on the west side of the lake, where it actually drops off a little more.
GPS accuracy is definitely an issue on those iphones! In this case it was way too far off for that to explain it, but yeah... I'm kind of sick of iphones. I guess it's a good thing mine went down the hole last night as I was googling "how to catch brown trout at night".... and a brown bit. I'm thinking it's time to switch to one of those Androids that is built for outdoorsmen. (it was mangled already-- the camera was busted and the screen was glitchy-- I was ready to replace it).
I've had trouble with Navionics on the south end of Iroquois, trying to find the humps by the island. Navionics is way off.
I've also had trouble on the south end of Dunmore.