Accelerated block generation
Block finding is random. The number of blocks in a period of time follows the Poisson distribution. If the average is 10 minutes per block, then the chance of at least 9 blocks in a given 24 minute period is roughly 0.086%. Not a lot but it can happen occasionally. Also, if momentarily the hashrate has increased without the difficulty catching up, the rate, and the probability, will be higher.
Yes, 6 confirmations that due to luck took, say, 30 minutes, are even safer than 6 confirmations that took 60 minutes, since the attacker would have had less time to find blocks in his alternative branch.