This is expected behavior due to how unique visitors are counted. The Referrer Performance report shows channel-level unique visitors that are not deduplicated across referral sources. If a single visitor arrives via Google on Monday and Facebook on Tuesday, they count as 1 unique visitor under Google and 1 under Facebook. Summing those rows gives you 2, even though it's one person. The Traffic & Engagement dashboard, by contrast, deduplicates visitors across all channels and shows the true unique visitor count. Both numbers are correct — they measure different things. If you need a total unique visitor count for a page, use the Site Performance report or the Traffic & Engagement dashboard totals.
