Does anyone know why does purepool ratio behave strangely? It should show 'Amount of your shares required until a block was found in average', but if you look at this one for example (and there are many more)
https://pool.purepool.org/main/miner/BS8B8E1tgT3jgtvtLZ6f7hWi9s5sxbohLU/ , it shows value of 11k, but this miner never broke 11k shares in one day, so it can't possible have 11k shares per block found.
The ratio is calculated not for one day, but for 3 days right now. One day wouldn't be enough for most smaller miners to give a fair impression.
The system takes the amount of blocks of the miner and the shares of the miner for three days and calcs the shares per block that way. Then it compares this number with the shares per block of the other miners (there average is "100%"). In your example, the miner needs 2.5x times the shares to solve a block. To make it more fair for everybody, this miner is getting more difficult hashtargets because of this.
Most of the time, this is only temporary. The system checks the ratio every hour and set it new if the miner looks good again.