Why is `patch` a separate ACL capability, instead of just governing it with `update`?

When the KVv2 PATCH support was added, it was made to require a separate patch capability.

The PR in question - Add HTTP PATCH support to KV by ccapurso · Pull Request #12687 · hashicorp/vault · GitHub - even says so:

Despite the fact that a patch operation could be interpreted as a type of update, the update capability will continue to function as it does today.

But it doesn’t explain why!

Does anyone know an actual reason for this?