Indexes have an associated field created_on in which are recorded the blockstamp at the time of the signature of a document.
I believe it should be called signed_on because the output generated on the same block does not have a value for the field. similarly the dividends dont produces a created_on value
they both indeed were created on a specific block but only a subset were signed. The difference explains better the presence of null records and is generally more specific
good luck renaming it though