Yeah it does happen though not often. You can HQ below the cap although it is really rare, HQ'ing is broken into tiers, I believe there is a 1% chance of HQ if you are below the cap or upto 10 above the cap.

Tier1: If you are 11 skill points above the item cap your HQ chance is around 10%.

Tier2: If you are 31 above the cap you break into Tier2 and HQ around 30% of the time.

Tier3: If you are 51 above the item cap, you have broken into tier3 and can expec to to enjoy a 50% HQ rate.

I'm no crafting expert though, and may have got some of this wrong check the tradeskill forums for more indepth guides into HQ Tiers.