Rear heater blower setting affecting the front

[autoque]

As strange as it sounds, when the rear heater's blower setting is increased, the front climate system also blows as strong as the rear vents, no matter how the front is set.
But the front climate system doesn't affect the rear, 'cause I didn't check the box beside "Use Driver's Settings" in iDrive.

I did not notice this until I sat in the back and tried. Anyone else with this weird problem?