Why no ghost?

Why is it that when hot lapping via the leader boards some times set have ghosts yet others don`t?

I think only top times get ghosts saved, something like top 500 or 100 but i think it changes based on the forza servers and the number of people playing
(observational hypothesis - not accurate)

it almost appeared until the start of this year that pretty much every ghost was saved. After the big bug fix/leaderboard car update earlier this year its like it only saves the very top times like ID says. I think that explains why there is some ghosts for peeps in 10000th place lol. If you look at the date the time was set its normally pre March/Aprilish?

I really don´t like this bug.
Example: Hockenheimring , 3-4 new good times and no ghost.
Racing against a ghost could help me getting faster.
Great...

I don't believe in ghosts