@TechKDB,
We do not have a document explaining the version differences other than the instructions on the product's firmware downloads that indicates what firmware is compatible with what hardware versions.
New versions can be released for a variety of reasons, such as a hardware change, chip manufacturer change, a change in production location, or a particular hardware version that could be made specifically for a retailer and given its own version. As far as incremental changes are concerned, the models share their firmware, and it is often an indicator of minor changes or optimiazations.