Mining Rewards, Difficulty, and Hashrate

Every time a miner successfully solves the proof-of-work problem and adds a new block to the blockchain, the protocol automatically issues the bitcoin reward plus any transaction fees included in that block. This payout is generated by the network itself, rather than a central authority. In addition, the Bitcoin protocol automatically adjusts the difficulty for mining blocks so that there is consistency in the system regardless of how much computing power is being contributed to the network. This is called network difficulty, and helps to maintain that about one block is mined every 10 minutes. 

Network difficulty is directly related to hashrate, the total computational power that miners contribute to the Bitcoin network, or how many guesses per second the network can make to solve the block puzzles. For example, if the hashrate increases due to more miners or more powerful hardware, blocks would be found faster than every 10 minutes, so the network increases difficulty in order to keep mining on schedule.

Fluctuations are experienced in mining for three main reasons:

1. Hashrate variability: The amount of computing power that miners contribute can change as machines go offline, join the network, or underperform.

2. Chance factor: Even if two miners have the same hashrate, one might find a block faster purely by chance. 

3. Network difficulty: As difficulty adjusts about every 2 weeks, it affects the likelihood of finding a block on any given attempt. 

All this being said, rewards tend to average out according to their share of total network hashrate.