Why is it so difficult to find info on rental prices that were released this week?

I checked some individual condos, and while some have gone up, others have come down. Here are some examples of median price psf from 2009 Q4 to 2010 Q1.
BLUE HORIZON 3.71 -> 3,57
CAIRNHILL CREST 4.86 -> 4,31
CARIBBEAN AT KEPPEL BAY 5.36 -> 5,28
CHILTERN PARK 2.28 -> 2,15
EAST MEADOWS 2.62 -> 2,57
HILLVIEW HEIGHTS 2.07 -> 1,97
HUME PARK II 2.00 -> 1,93
PARK INFINIA 4.59 -> 4,51
SIMSVILLE 2.57 -> 2,55
SYMPHONY HEIGHTS 2.24 -> 2,21
UE SQUARE 3.37 -> 3,25
Obviously there are hundreds of condos in SG but I only checked the ones I have heard of.