If you want to call it a fault. One instance can't cover all, just the slice its users interact with.

Your instance was created this year, if that's what you mean. My instance is a year older:

Also, defederation would impact this as well. For all I know the missing posts are on lemmy.grad (for example).

Edit: actually it's definitely the age of your instance.

I got curious and decided to view my profile from feddit.org and it shows less than half of my total posts/comments

I like this actually, I look a little more well-adjusted lol. It also shows just how much effort I was putting into commenting on things early on to break the seal

it's like @Xyre@lemmus.org described. old content does not get federated unless there is new activity on it and someone on your instance is subscribed to the community it happens in.

this isn't anyone's fault, it's just a matter of the instance not being around that long that some historic content will be missing.

With the way Lemmy is primarily used this will largely not be an issue anyway. None of my old posts would show up to you either way.

Edit: aside from being in the aggregate — they're not going to be in your feed either way