What happened versus why it happened
On-chain analytics can tell you, with a high degree of confidence, what happened: which address sent what to which contract, when, and what events resulted. What it cannot tell you is why. A large token movement might mean an investor is losing confidence and heading for an exchange, or it might mean funds are being moved between two wallets the same person controls, or rebalanced into a different strategy, or transferred as part of a completely mundane operational process. The chain records the action, not the intent behind it, and there is no on-chain field anywhere that captures motivation, so any explanation of 'why' layered on top of the data is inference and interpretation, not a fact the chain itself provides.
This matters because on-chain analytics is often presented, informally, with more certainty than it deserves. A chart showing large holder movements or unusual activity spikes is genuinely useful as a starting point for investigation, but treating its implied explanation as confirmed fact skips over a real gap. Off-chain context, news, communications, business decisions, is frequently what actually explains an on-chain pattern, and none of that context is visible to an indexer no matter how well built it is.