Yeah, the spelling out of the numbers was the first thing I tried. Here is a screenshot of what is happening. Spoken "F ONE" gets translated to F1 and the F1 command is executed. Spoken "F TEN" gets translated to F10 BUT the F1 command is executed!
If I use a non-numeric word (I use MAP because F10 also shows the map when a menu isn't up) and "F MAP" works perfectly to invoke the F10 command.