560SEL Defrost Blows Hot All The Time

On my 560sel - heater and A/C functions with Climate Control work normally. However - if I get a mist on the inside of my windshield and put the defrost on - the blower goes on high and real hot air blows onto the windshield. OK for winter - but in the summer - a lot of extra heat. Turning off defrost allows normal operation of CC. Is this normal operation or is there a function of the ccu that determines the defrost air temp depending on if the A/C is on or the heater is on? On all my other non-Mercedes cars - if you put the defrost on - the temp is the same as is coming out of the vents - whatever you have it set to.