It seems the IR receiver circuitry kind of adapts or something... I guess that's caused by the circuitry inside the IR receiver.
Yes, IR receivers automatically adjust their sensitivity. If you look at your datasheet it will refer to this as "automatic gain control" or "AGC" in the block diagram. You can compare this to Vishay part TSSP4038 which does not adjust its sensitivity (it's "fixed gain").
TSSP4P38 is what you're looking for if you want to do distance detection. http://www.vishay.com/docs/82474/tssp4p38.pdf
. This is $.95 at Digikey.
See also http://www.vishay.com/ir-receiver-modules/
for a nice matrix list of different IR receiver types.