Edit: After rethinking, all three of the batches that got down to 1.006 have been all grain batches (I was doing extracts before and getting to 1.010-1.012) so maybe this played a role in the lower attenuation?
Most likely, this. Extract is usually, to the best of my knowledge, targeted at a balance of attenuation and body. This may also play some role in the difference for your Krausen.