Reach and impressions mean different things generally, but the gap between them reads differently depending on whether you're looking at a carousel or a single image — and conflating the two readings leads to a wrong conclusion about what's actually happening.
Why carousels structurally produce more impressions per viewer
A single image has one surface to view — one impression per person who sees it, generally. A carousel has multiple slides, and swiping through several of them can register as multiple impressions from that same unique account. That means a carousel's impressions-to-reach ratio being higher than a single image's isn't automatically a sign of anything unusual — it's partly just how the format works mechanically.
What the gap actually tells you, format by format
For a single image, a large impressions-to-reach gap usually means the same accounts are seeing it repeatedly over time — appearing in multiple surfaces, or people revisiting it. For a carousel, that same gap more often means people are swiping deeper into it in one sitting — genuine engagement with the content itself, not repeat exposure over days. Same metric, different underlying behavior, because the formats generate impressions differently.
Why this matters for judging carousel performance
If you compare a carousel's raw impressions number directly against a single image's and conclude the carousel "reached more," you're comparing numbers that aren't quite apples to apples — the carousel's number is inflated by its own multi-slide structure in a way the single image's can't be. Reach is the fairer cross-format comparison for "how many new people did this find," since it's counted the same way regardless of format.
Using swipe-driven impressions as its own signal
Rather than comparing a carousel's impressions to a single image's, use the gap within a carousel's own numbers as a depth-of-engagement signal specific to that format — a carousel where impressions barely exceed reach means most viewers dropped off after slide one; a carousel with impressions well above reach means people are swiping through, which is closer to what carousel hook and slide design is actually trying to produce.
The practical takeaway
Don't default to "carousels get more impressions so they're performing better" — check reach for the cross-format comparison, and use the impressions gap as a carousel-specific read on how many slides people are actually making it through, which is a different and more useful question than raw impression count alone.