That said, I imagine they do have the data, at least in some form, for accounting reasons.

Looking at what it said Apple was requesting, it's entirely feasible that they don't. They may well not keep logs of every price change every single game historically has had (although they could potentially reconstruct that by looking at sales transaction histories), and they may lose a degree of information when recording transactions depending on how its encoded.

It may also include microtransactions that Valve doesn't see at all and only serves as a storefront for purchasing premium currency (or requires a company to report microtransaction revenue from steam users).