Did you let the engine come up to temperature? I have an AEM and have used them for a few years now. When the gauge is cold it sometimes takes 30 seconds or so to get a reading. Also, on my 440 I don't have a choke, and I keep the throttle cracked a little and high idle it myself for a minute or so when I start the car. During this warm up time the gauge typically reads pretty lean, like high 15s, most likely due to incomplete combustion until the engine gets some heat in it. It probably takes until I've got about 130 degrees in the engine to get it to settle in.

Now I need to pin those needles, got to feel that heat
Hear my motor screamin while I'm tearin up the street