Would someone please describe why some categories begin with an underscore?

Unknown

 I have a "_Div Income" as well as a "_DivInc" category, and I don't understand the apparent duplication.  There is also a "Div Income" category without the underscore, but I may have created this some time ago.

NotACPA

_DivInc is a Quicken-created category ... that Q uses to record your dividend transactions.

_Div Income (note, in particular, the space) must have been created by you, as was 'Div Income".

Combine the 2 non-Q created categories into _DivInc.

While you CAN, as you've discovered, use the underscore preface ... in general all of those categories are created by Q itself.