At intervals throughout each day your swim totals and position in the challenge are calculated. Your total includes all the swims you have logged.
If you happen to log a swim inbetween these intervals then your name will appear at the bottom of the page, it's a bit like a waiting area and let's you know that your swim has been noted and will be included in the next update today.
When the next update occurs all those listed in the waiting area at the bottom of the page get included in the 'official' list that you see above this. The challenge positions are adjusted at this time.
If you log a swim for a previous day and view the attendance page for that day you will see your name in this bottom waiting area. It won't be moved up to the official list. Your challenge position for that day will not be influenced by that swim. Only your swim total and challenge position for today will be affected.
Likewise, if you delete a swim for an older date then that day's challenge remains unaffected by the decrease in your swim total. The next time the system calculates your total and challenge position for today will be adjusted.
Hopefully that makes sense, if you'd like to understand it better, use the feedback option in the menu option (top right) and tell us what it is you'd like more information on. Enjoy!