Great question! I've wondered about why that doesn't hold true as well.
I have some loose leaf green teas that are very sensitive to steeping
time and in as little as 5 seconds beyond begin to taste bitter. But,
the second infusion tastes fine with no bitterness. I don't know why
that is. Maybe there is a chemist that could explain why that is (not
sure how much I care though) ;-)

Usually, the second infusion is for the same length of time (3
minutes), and a third infusion for an additional 10 - 15 seconds.
Sometimes I do a fourth infusion and let it steep an extra 30 seconds
with no bitterness. I am doing fewer fourth infusions.

....mmm, enjoying my Adagio citron green... ohhhh... smooth.

Also, I am in the habit of first infusions in the afternoon or evening
being 20 - 30 seconds and throw away in order to make decaf on the
second infusion.