Get Historic Number of Stake Pools via cardano-db-sync

I want to understand how the number of Cardano stake pools developed over time. I am running cardano-db-sync. I thought the pool_update and pool_retire tables would give me this information.
My idea was the following: Whenever I see a hash_id for the first time in the pool_update table I would count that as a stake pool being created. Whenever I would see a hash_id in the pool_retire table I would count this as a stake pool being removed.

However, I come up with a higher number of stake pools when compared to what I can see on (~2,000 pools).

How can I get the correct numbers over time?