It's also important here to make a distinction, and it's not a trivial one, between a combo box and a dropdown box.
A dropdown box allows you to make a selection from the dropdown list - that's it. You may, in some implementations, get away with using the first letter of a choice to get to it quickly, but that's the equivalent of arrowing down to it. You can't change it in any way, only select it.
A true combo box, hence its name, is a merger of an edit box and a dropdown list. You can type in a combo box and it will accept the input. Some do edit checks afterward to ensure that what's there matches one of the items in the dropdown list (most common), while others intentionally don't, so that the end user can essentially give an input that is not one of those things.
I've actually never paid close attention to whether dropdowns and combos are announced differently, but I have to believe they are since their behaviors are, while related, distinctly different.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back