There's an bug with dag generation and share submission. After about 4 hours of mining, Excavator generates a new DAG cache, and then the next dozen or so shares are rejected with errors, "Share above target" or "Job not found"
Actually, it's a lot more than a dozen shares. Checking the output, 791 total shares, 720 accepted, 69 were rejected with "Share above target" - that's a 8.72% failed share rate.
2 were rejected with "Job not found".