@ditol data is expensive and by follow relationships, I mean across your instance.
Also, boosts matter, that's more than likely why you only see a limited amount of posts. They were either boosted by someone on your instance or the posts are pinned to the users profile. Pinned posts seem to be held onto.
If person 1 on server A follows person 1 on server B, then any users also on server A will be able to see those posts.
Instances have the option to delete remote content after a set period of time to save data as well.
These things are all done to save data and keep costs low. If every instance grabbed every post and held onto them, Fedi wouldn't exist.