It's not only a difference in masculine and feminine forms of the same word. In the bible the term used is דברים, from דבר (Davar), loosely translated as a "the items that were spoken about". In Chazal, the term used is plural of דיבר (Diber), used as a noun to mean specifically the word of God as transfered by a human prophet, as in Jeremiah 5,13:
וְהַנְּבִיאִים יִהְיוּ לְרוּחַ, וְהַדִּבֵּר אֵין בָּהֶם; כֹּה
The exact reason Chazal switched the term is difficult to say, but suffice to note that their Hebrew was rather different. Already in the time of the Mishna, דבר came to it's more modern meaning, i.e. "a thing / item", without the context of being spoken, e.g.:
עשרה דברים נבראו בערב שבת בין השמשות
This may suggest that Chazal wanted to emphasize the divine origin, moving away from a term that at their time meant "The ten items", moving towards something closer to the original meaning of ten divine utterances.