Bug in the client, or the client hasn’t downloaded your newest follow list, you follow a new person, it appends it to an older version of your list, and it makes an incomplete list that looks newer than your full one.

If that made any sense.

Relay connection changes most likely.

Happened to me, though I've found most of my follows now. My old phone died, and when i got on nostr on new phone, i had to start over.

Use https://metadata.nostr.com/ to backup your profile regularly.