"the middle" is not the perfect definition to explain where we put the mic but in fact it can explain the 100Hz dip.
We put it at the listener's spot (which in this case may not be the perfect place - it changed after
. I also know the room is not perfect however.
These measurements are just provided to give an "idea" of how things can sound. It's not done with the most accurate tools (but with tools that any home studio amateur can acquire) and must not be taken as a "reference".
Thank you for your consideration