UB, your analog gauge on the outside edge of the drum that you had at grate level doesn't see the same heat as the center of the barrel. Your gauge is probably just fine.
That's why several on here use or are experimenting with diffusers. The center to outside edge heat differential is very common on UDS style smokers.
I always use a Maverick digital thermo to monitor the center of the drum heat and the meat. I rarely even pay attention to the analog gauges on the outside edge anymore as mine have only 2.5" probes. Once my drum hits 300 degrees, the center digital thermo and the analog are within a few degrees of each other.