The 'audience' denominator varies: engagement / followers (audience-based), engagement / impressions (reach-based), engagement / reach (per-person-based). Each gives a different number; comparing the wrong denominators across platforms or tools is a common cause of confused dashboards.
Engagement rate is a quality signal more than a volume signal. A small account with 5% engagement rate often outperforms a large account with 0.5%, both for organic distribution (platform algorithms favour engaged content) and for monetisation (advertisers prefer engaged audiences).
Benchmarks vary by platform and follower count. Instagram for small accounts averages 3–6%; large accounts 1–2%. TikTok runs higher because the algorithm distributes to non-followers. LinkedIn averages 2–3% on organic posts. Use your own historical baseline as the most important comparison.