I assume you mean a Lee Auto Disk powder dispenser, not the 'Auto Prime'...?
This is most likely the result of powder settling in the hopper; particularly when using it on a turret press, the powder will tend to settle more densely in the hopper due to your repeated turning & jostling of the press as you load rounds, & this will increase the throw weights a bit. I've found that dispensing up to 10 'test' throws will help make sure that the powder is settled in before you start dropping 'real' charges; that should ensure more consistency, although you may find that it drops 6.9gr more consistently then your desired 6.6gr...
Although I'm not using it on a turret press, I've run into similar behavior; I eventually just moved to the next smaller disk, which actually throws the charge weight I was looking for very consistently.
I've read of other people resorting to installing some sort of baffle in the powder hopper to reduce the weight/pressure of the powder column right over the disk opening, but that didn't seem to be necessary in my case.