Well, from what I understand, it's because the hormones don't directly affect your voice. Testosterone causes your vocal chords to thicken, and thicker vocal chords lead to a deeper voice. So for FTM transgenders, they will get a deeper voice because the testosterone can still thicken vocal chords regardless of when it is taken. For MTF transgenders, as you naturally produce testosterone as a male, your vocal chords will have already begun to thicken to produce a deeper voice. Unless of course, you began the hrt pre/mid puberty and haven't been exposed to as much testosterone. While it doesn't exactly change your voice, it stops your vocal chords from thickening as much as they would naturally due to exposure to less testosterone. But if you're already through that state in life then nothing will happen, so your original conclusion isn't exactly wrong--merely only part of the equation.