Mine works the same way: the A/C outlet temp is constant; the blower speed changes with cabin temp. However, the heater does change outlet temp according to cabin temp.
I suspect that later years had a blending function for the A/C. Perhaps someone can chime in here.
Ed in San Jose '97 540i 6 speed aspensilber over aubergine leather. Build date 3/97. Golden Gate Chapter BMW CCA Nr 62319.