The Bitcoin network is designed to produce a new block every 10 minutes on average. However, there is a natural variance in the time it takes to mine a block, and this can lead to periods of time where multiple blocks are mined in quick succession. This is known as a "block time variance" or "block time clumping".

There are a few factors that can contribute to block time variance. One factor is the hash rate of the network. The hash rate is the total amount of computing power that is being used to mine blocks. When the hash rate increases, the difficulty of mining blocks also increases. This can lead to longer block times, as miners have to spend more time and resources solving the cryptographic puzzle required to mine a block.

Another factor that can contribute to block time variance is the distribution of mining power. If a small number of miners control a large percentage of the network's hash rate, they may be able to mine blocks more quickly than other miners. This can lead to periods of time where multiple blocks are mined in quick succession.

Finally, block time variance can also be caused by random fluctuations in the network. These fluctuations can be caused by a variety of factors, such as changes in the number of miners online or changes in the network's infrastructure.

While block time variance can be disruptive, it is not a security concern. The Bitcoin network is designed to be robust enough to withstand even extended periods of block time variance. As long as the network continues to produce blocks at a rate of at least one every 10 minutes, the Bitcoin protocol will continue to function as intended.

In the case of the F2Pool mining pool, they were able to mine six consecutive blocks due to a combination of factors, including a high hash rate and a favorable distribution of mining power. While this was an unusual occurrence, it does not pose any serious security risks to the Bitcoin network.