1989 Chevrolet Silverado transmission

My guess is you're checking it cold with the engine not running. That is not accurate or the correct way to do it. Transmission fluid will slowly run out of the clutch packs and some of the passages and make the fluid level appear to be much too high.

Transmission fluid is always checked with the engine running and warmed up. All the passages, clutch packs, and servos will be filled up, and the remaining fluid in the pan is the level you're checking.