Shadow nailed it.
The easier it is to disappear, the less of it they will let you keep in it. Cash is usually allowed at 10% to 20% of the rating. If you had a safe insurable at $250,000 in assets, but it was being used at a check cashing store, it may only be insured at $25,000 to $50,000.
This varies a bit from insurance company to insurance company. In some cases you can get coverage that exceeds the rating. Firearms are a good example. They have serial numbers, involve federal laws, and usually get a lot of attention from the police when they go missing. This is why a number of gun shops get away with using regular gun safes (or no safes at all sometimes) for their inventories.