Three factors are important, and one factor less important, when you need to understand why earnings both go up and down:
This is the time it takes to find a new block on average. On the ETH network, at the time of writing this explanation, it was about 13.8 seconds.
This is how much ETH there is in each block that gets found, which is then rewarded to the miner (pool) that finds the block. This reward goes up and down but normally between 2.0 and 2.2 ETH. We will use 2.1 ETH in our calculations below!
The more miners on the ETH network, the less reward will go to each miner. When it's profitable, the network hash rate will go up, and when it's not profitable, the network hash rate will go down as miners stop mining. At the time of writing this, the network hash rate on ETH is around 831 TH/s
There isis also a bit of luck involved, but this will equal over time. There is the luck of finding shares - if you are unlucky, you will find fewer shares - and get less paid by the mining pool. At the same time, if the mining pool is unlucky, the mining pool finds fewer blocks. The higher the hash rate you have as a miner, or the pool has - luck will be less a factor for your income.
Let's see how much ETH you get in 1 day of mining, with 1 MHs, based on two different network hash rates:

Blocks per day: 86400 sec/day / 13.8 sec = 6261 blocks/day
Block reward per day: 6261 * 2.1 ETH = 13148 ETH
Network hash rate: 831 THs = 831000000 MHs = ~0.0000158 ETH per MHs per day

Let's say the network hash rate goes down to 580 THs:
Network hash rate: 580 THs = 580000000 MHs = ~0.0000227 ETH per MHs per day
The price of ETH goes up and down based on the market. Let's do some quick examples:

With a network hash rate of 831 THs:
ETH at $3800: 0.0000158 * 3800 = 0.06004 USD per MHs.
ETH at $2800: 0.0000158 * 2800 = 0.04424 USD per MHs.

With a network hash rate of 580 THs:
ETH at $3800: 0.0000227 * 3800 = 0.08626 USD per MHs.
ETH at $2800: 0.0000227 * 2800 = 0.06356 USD per MHs.