Try measuring higher resistance carbon resistors at RF. The shunt C of somewhere under 1 pF becomes the issue as frequency is increased. My first investigation into this was with the ancient HP(Boonton Radio Corp design) 250B RX meter back years ago (maybe in the '80's). The lead inductance is swamped out by this shunt C. The 250RX meter could indicate the Cp down to tenths of a picofarad and the Rp up to 100 k. I still have the old beast and use it and an old GR-821. I was using them to investigate the dielectric properties of PVC at RF when there was a discussion about how lossy it is at RF (not what I found). I didn't see the value of chucking a piece of the stuff into a microwave oven at 2.45 GHz when we were concerned <30 MHz. I wound up figuring out how to determine the dielectric constant along with the loss tangent of the PVC and for other materials.
