UHF RFID: Why are read range (forward) and backscatter range curves different?
You don't give much detail on your experimental setup or indeed what you've exactly measured, and you mention these figures are calculated.

In the first case I think you're measuring the ability of your transmitting antenna to couple energy into the receiving antenna, and you get the classic two peaks you'd expect with two loosely coupled [somewhat] tuned antennas.

If I understand your second graph what you're showing is the sensitivity of your antenna and receiver to receive backscattered signal, so this is the merely response of your receiving antenna and amplifier. There's no coupled system and you get a single peak corresponding to the response of your [somewhat] tuned antenna.

PVC and Plywood do not have permittivity near one, it's more like 3. Your other materials (apart from the ferrite) are mostly air and have a permittivity of near 1. (depending on the type of PVC, PVC can also interact in odd ways at microwave frequencies) 

-- edit -- 

Actually you can see the permittivity effect quite nicely, for your received signal (2nd graph) it lowers the tuned frequency, and for the coupled graph you can see it affects the position of only one peak (the slight motion of the second being due to compositing it with the rapidly falling tail).