SeventhLord it's because the older ones expire, 21/3 = 7 so the last 3 are 7 days old so they expire
Fast passess expire early
Also keep in mind the day change over is based on CHINESE time. For me, east coast Canada thats about 2 pm in the afternoon which makes it very easy to miss the day change over if I have a busy morning, so don't think the expirey date is based on your own 24h estimation.