sha-1 check in rev-list --verify-objects redundant?

Hi,

"git rev-list --verify-objects" calls parse_object() on all except
commits. This function in turn does check_sha1_signature() which
rehashes object content to verify the content matches its sha-1
signature. This is an expensive operation. I wonder if this is a
redundant check. --verify-objects is called to verify new packs.
index-pack is also (always?) called on new packs, and index-pack
hashes all object content, saves the hashes as signature in pack index
file. So they must match. Am I missing something here?
-- 
Duy