You see this (material differences) when you look at cars. Some look red, some blue, etc. That is the different chemical compounds in the paint absorbing or reflecting different frequencies of visible light. Same thing with radars. Different frequencies see different materials better or worse. Or when you look at a car in the sunlight you notice "bright spots" where sunlight reflects off a chrome strip, curve of a fender, windshield, bumper, etc. That is a physical feature that creates a "specular" reflection in the visible light spectrum. Same thing happens at radar frequencies. You can make most of the vehicle disappear, but some features stand out at some frequencies.
VHF radars are relatively low in the frequency bands as search and track radars go. Used to be antennas for these kinds of beasts were huge. Think building/bridge sized. Some of our early UHF radars are the size of small office buildings. (and UHF is higher than VHF) The other problem with lower frequencies is resolution. Used to be you couldn't get a fire control grade track with lower frequencies.
I guess modern antennas and digital processing techniques have taken away some of the negatives of lower frequency operation. You can't build an aircraft that is invisible to every spectrum. As I understand it, the 35 is only supposed to be partial stealth. Basically only from the front aspect (hey it is an attack aircraft). The other consideration is that it only has to be stealthy enough to delay detection such that it can get close enough to launch standoff weapons and take out the radar. So that's the battle, can the air defense system detect it and vector interceptors to it before it is close enough to launch on the radar...
Contrary to popular misconceptions, physics is a b!tch. The effective aperture size of an antenna is basically a function of wavelength and physical size (with some limited exceptions that usually are inherently narrow banded in nature). It is still why you see HF and even some VHF OTH radar systems buried deep inside the air defense perimeter of any given country.