The person who signs
Spans of control, delegation, absence, and the approver who has no way of knowing.
Span of Control and Attention
An approver's capacity for review is finite and countable. Organisational design decides it, and organisational design is made without anybody counting.
What the Approver Actually Knows
Review depends on independent knowledge of the week. Most approvers have very little of it, and what they do have is rarely put in front of them.
Delegation and the Chain of Signature
Approval authority gets passed on, formally and informally. What the record should say, what it usually says, and where the chain breaks.
When the Approver Is Away
Absence is the largest single generator of unreviewed approvals, and the cover arrangement is usually made after the queue cleared itself.
Line Manager or Project Lead
In matrix organisations two people have a claim on the approval and each knows half of what is needed. Most implementations pick one and lose the other half.
Approving Your Own Hours
At the top of every chain somebody approves themselves. It is unavoidable, it is where the hours are most valuable, and it is almost never designed.
The Manager Who Reads Them
In every organisation two or three approvers do this properly. What they have in common is mostly structural, and most of it can be copied.