it takes the number of WIP from the last 10 days (one per day) and orders them in ascending order rather than the order of days (2, 3, 3, 4, 4, 5, 5, 5, 8, 8).
...
WIP: 2 - 3 - 3 - 4 - 4 - 5 - 5 - 5 - 8 - 8
The same principle applies to 50%, 5 data points reflects 50% od 10 data points so it rounds down to 5.
WIP: 1 - 2 - 3 - 3 - 3 - 3 - 4 - 8 - 8 - 9