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(A) You are overestimating your ankle mobility with the knee straight (are you just glancing down, or have you taken picutres of yourself?)
(B) You are letting your knee drop inwards doing the straight knee stretch, which essentially creates the illusion of ankle mobility when it simply bypasses all the restriction.
Anatomically, it wouldn’t make sense that, if you were using the same ankle movement for each position, you would have better dorsiflexion (toe-to-shin ROM) with your knee straight vs bent. In fact, it’s usually quite the opposite. Allow me to elaborate.
Your soleus muscle, the deeper of the big movers in your calf, only crosses the ankle. This means your knee position doesn’t really affect your ability to move your shin forward over your foot. Your gastrocnemius muscle, however, crosses both the ankle AND the knee. This means that if your knee is straight, it puts more tension on the gastrocnemius which may limit your ability to bring your shin over your foot.
So yeah, I have to guess that there is an inconsistency between your positions when doing each stretch. Your best bet would be to take pictures of your lower body from the back and sides when doing each stretch and post them so we can confirm that.