Because they support different business classes – e.g. the UK Private Motor dictionary (PMC) supports one product and holds objects and properties for that business class. The UK Commercial dictionary (CML) has a more generic, flexible structure because of differences in the size and complexity of commercial products.

Schema dictionaries can be generated from industry-wide messages or bespoke schema definitions, holding just the data needed for a specific product.
https://www.polarisukltd.org/support/productwriter/productwriter-toolset/frequently-asked-questions/why-are-the-dictionaries-structured-differently
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