Okay so, hopefully this makes sense!

Using this achievement as an example, you likely hit 750 long ago. When we add a new tier to achievements like this that track a task, you have to do something to trigger the latest tier. As you saw with this, you were awarded that tier once you grabbed a new item that qualified for the achievement. It just has to "catch up" and trigger that new tier.

So even if you had say you ended on 1001 last Fireside, you'd still have to grab another item before getting this tier (even though the tier is 1000). Just to trigger it!