No, it's not just you.
I noticed this more on the baritone (where it's D and D7), and a quick googling revealed that guitar players ask the same question. I do hear the difference in a chord progression (when you go from G7 to C again vs. from G to C, or from D/D7 to G on the baritone), like Jollyboy mentioned, but when I just play the two chords back and forth, I hear no clear difference, even though there is an extra note. I hope some day I will, in my own playing. I imagine it's trainable. But yes, we're not the only ones who noticed this. (Phew!)