Level 2 are a lot less complicated that Level 3. It is easier for casinos to verify the legitimacy of ID documents for level 2, than having to validate the authenticity of documents that are needed for Level 3.

You have to remember that these source of income documents are provided in different languages and that they need specialists third party companies that are efficient in those languages to verify these documents, so it will take a lot longer to authenticate these documents.

Good luck, if you source documents are not in English!