If he's running without V-sync, his GPU should be holding it's base clock at minimum, no? To the OP, can you take a screenshot of your settings in game, as well as a screenshot with MSI Afterburner overlay? My GPU will hold clocks with a 100% load, but it won't when I play Quake Champions, which I play at 720P, because my monitor can only overclock to 75Hz at thes res...which leads to a game that's CPU bound instead of GPU...my CPU can only push 130ish FPS, while my GPU is barely doing anything. In that scenario, my GPU clocks will jump all over. Anytime V-sync or any other form of frame limiting is happening, the clocks are going to bounce around. Basically, unless your GPU is fully loaded, it won't clock all the way because it doesn't need to.
EDIT - disregard...I was seeing a card with 900MHz base clock...but I then just searched "280X 870MHz" and there are several options...you're good, your card is behaving as it should!