It is the driver package, I have the same issue. The card is not actually running at that speed.
On top of showing idle clock speeds, my card also started reporting GPU usage locked at 16% with 17.7.2 and the behavior was retained after rolling back to 17.7.1. The newest 17.8.X driver is better but still incorrectly reports usage for me, unless I have "Unified GPU Usage Monitoring" enabled in MSI Afterburner. Even then, only Afterburner reads correctly.
I installed 17.4.3 instead and then Windows reverted my card to 17.1.1 as the "recommended" package. Everything is reporting properly for me now.
EDIT: Crimson 17.5.2 also worked well for me and is reccommended by others, but it is non-WQHL if that is of concern and I figured I would...