Yep, they're funny that way. The temp difference from center to edge changes as the drum warms up.
The temp gradient will be different when you throw a big ol' slab of cold meat on then it will change as the meat gets up to temp.
You just kind of have to cook enough on it to get a feel for what the guage is telling ya but that takes time. Depending on what I'm cooking, I'll use a digital probe. But with a 15 pound packer on the grate, there's not much room for a block of wood and probe.
A long thermo through a bolt is an option to get center temp though.
I wish Teltru made a thermo with a 12" stem.