This has already been debated to death.
AFAIK, no one has disassembled the Q701s to see if they're actually identical. Yes, I know the service manual says they should be. It's possible the Q701s have some different damping on the
back of the driver.
I was skeptical too that they could sound different, for the same reasons you are. It doesn't make sense that they would sound different. I didn't want to keep going over the debate again and again, so I ordered a pair of K702 to compare directly to my Q701 myself. The pair I compared back to back, A-B, on the same setup, etc. DID have all the changes we've been describing between about them (I think you've already seen
that thread). I was surprised that the difference between them was as large as it was. I was expecting a minor difference that could be attributed to variation in production, but the difference I heard sounded intentional.
I don't know why my pair was so different. I'm welcome to suggestions. I'm not a huge believer in burn-in. The Q701s were bassy and warm for me out-of-the-box. I was surprised at how much bass they had the first day I put them on. Both AKG pads look pretty new and identical. I doubt any subtle changes between them could add several dB to the bass.
I know others like Tyll have heard them the as sounding the same.
That to me is the confusing part. If he heard the pair I heard, I guarantee he would have wrote a completely different comparison. The only thing I can think of is AKG has had some variation in the x70x over time (intentionally or unintentionally?...) That could explain why some sound the same, and some sound different....
I could probably take apart the Q701 and look at the insides. I've taken a K400 apart and the Q's look similar. But the Q701s are my babies, and I'm scared!!