Pretty sure they are saying that if you have 10 days PTO and you use one of them when sick, you no longer get a full two weeks' vacation as you'll have an uncovered day. With a full 10 days, I could clock out Friday evening, get on a flight to my vacation destination, catch a return flight the afternoon of the 19th and be back to work on the 20th. With only 9, I either need to work until next Monday and get on the plane that night, or cut my vacation short to fly back in the 16th and work the 17th. You effectively lose up to 3 whole days of downtime on vacation for being unable to work due to illness once a year.