“Reject rate” and calculation
The mechanism for a mining pool is that the pool continues to send out calculation tasks to miners and miners submit results to the pool. But sometimes when network delay happens, miners’ submissions during the period won’t be admitted by the pool as it has gone invalid. Reject rate is invalid submissions against the total.
Sudden rise of “Reject rate”
It may be caused by internet instability, which doesn’t happen often. Please check your internet connection if it lasts too long.
“Luck” and calculation
Luck = Mining Difficulty/Work Shares of A Pool. The less shares the pool contributes to mine a block, the luckier it is. Luck is an important factor for mining and it fluctuates frequently. But in the long run, the average luck will be close to 100%.