In Bitcoin, once someone succeeds in mining, the competitors stop mining immediately and engage in the next block mining. This is because there is no reward for those who come in second place. In the case of second or third place mining, they are slated to be orphaned and thrown out. If they fail to do their best in the next 10 minutes, they face the misfortune of being orphaned. Accordingly, competitors have no time to look back.
The average interval between one block and the next is 10 minutes for Bitcoin and around 10 seconds for Ethereum.
With Ethereum, there is a reward for those who come in the second or third places, although there is no reward given from fourth place. As a result, if somebody declares his or her success in mining, there is no reason to stop. For the main block that succeeds in mining in first place, 5 Ethers equivalent to 1 are given as a reward. In case of being included in the main block as the qualification of uncle, 4.375 Ethers equivalent to 7/8 are given so that it is not so bad.
Uncle mining refers to what people continue mining with a target of the second or third place, even if the first place was decided. We can estimate many reasons for allowing uncle mining.
Firstly, 10 seconds, the interval between block to block is too short. Within 10 seconds, they have to mine, herald the mining news and confirm it. So miners with a slow network may fail to hear the mining news within the 10 seconds.
Secondly, making a success in mining within 10 seconds, not 10 minutes, means a low level of relative difficulty. In case of a low level of difficulty, the number of nodes solving problem can be increased. However, if the nodes become orphaned frequently, they think that it is more profitable to abandon mining and move to other coin. Accordingly, granting considerable reward to second or third places becomes an incentive to getting miners.
Main blockchain is created from father block to child block, and further to grandson block, and great-grandson block for generations. In case that two miners succeed in mining by a close call in the child block generation, they are brothers. Of them, the elder miner who succeeded in mining faster possesses the child block. But, the elder miner may not know the younger miner's result. To do so, 10 seconds are too short. So the elder block cannot take care of the younger block.
As one generation has passed again, the grandson block starts. The grandson block knows that there is an uncle block that has been left without being taken care of in the above generation. Accordingly, in case of including the uncle block in the grandson block, the uncle block is to get a reward equivalent to 7/8.
There is another story. Let's suppose that there is a laggard miner who has belatedly solved a problem in the child generation. The laggard miner knows that the main block has just passed the grandson generation. Viewing it in terms of time, the miner made a success in mining of the child generation belatedly when he reached the grandson generation of the main block. From the viewpoint of the great-grandson block, it looks like a block being left without a place to go in the grandson generation. Namely, the block neglected by the great-grandson is not a nephew block actually, but call it a nephew block for convenience. Accordingly, in case of including the nephew block in the great-grandson block, the nephew block gets a reward equivalent to 6/8.
In this way, the great-grandnephew gets a reward equivalent to 5/8. The great-great-grandnephew gets a reward equivalent to 4/8. By doing so, a certain block gets a reward equivalent to 1/8.
As a result, we should really get to know the genealogy. Of course, we can get the reward even if we don't know the genealogy.