Imagine you do a little mining for a couple hours in the morning. Later you mine a couple hours in the evening. We find a block and both the morning and evening work is paid. Later we find another block and only the evening work is paid, as the morning work is no longer among the 10 latest shifts.
Now that the last 10 shifts can be accomplished in roughly 24 hours, it seems to me likely that a shift not paid could scroll off the list. In other words, the work is never paid. Am I thinking about this incorrectly?