Climb415 - I believe the website releases dates as they receive cancellations or those that give up their dates they received in the lottery.
Yes! Anyone who won a slot has until May 1 to pay the $15 per hiker fee to keep their slots. So, some people are paying for some, but may release a slot or two as group members back out, or else the completely cancel their won slots. When this occurs, recreation.gov subsequently releases those slots back to the system at a random time within the next 24 hours.
Those are the slots that appear every so often. They are available to the first person to grab("book") them and pay the fees. After they "book" the slots, they are not available to anyone else; the website gives you 15 minutes to complete the payment. If not completed, they are again released immediately so others can "see" them again.
On May 1, all the reservations that are still not paid up will be released, starting at 7 AM, at random times. There should be a lot more popping up that day.
Here are links to Recreation.gov and the permit reservation page: DAY USE of Mt. Whitney Zone
Mt. Whitney Trail Overnight Permit