Well, I just remember how Ron Moore explained it when asked why the female military officers were addressed as "sir" in BSG. Of course, they even considered addressing the female President Roslin as "Mr. President" but there were too many objections raised by the other writers over that.Net sources tend to disagree: "ma'am" appears the default expression for female officers and warrant officers, while "ladies" may be used for plural.
But Starfleet could go for simplicity. Or for diversity, as Janeway makes it sound like a personal preference first and foremost.
Timo Saloniemi