The Microphone icon's center point indeed shows exactly the positions where the microphone arrays were put when capturing the impulse responses for a MIR Venue.
Most rooms exhibit certain acoustic penomena like resonances and/or frequency cancellations, or flutter echoes along their axes of symmetry. There's a reason why the best orchestral halls exempt these areas from the audiences' seating. ;-) Consequently we tried to stay outside these areas whenever it seemed necessary.
It's quite easy to overcome this real-world facts, though, by simply moving MIR's Microphone Icons towards the symmetry axis.
HTH,
/Dietz - Vienna Symphonic Library