khotkar said:
"Why there is more variation in nwell resistor?"
nwell is a real, lightly doped, i.e. relatively high-ohmic semiconductor material, as such has a much higher voltage dependency and a distinctively higher temperature dependency than, e.g., polysilicon, which is a highly-doped, relatively low-ohmic, thus already degenerated semiconductor, whose conductivity mechanism is caused by less semiconductor-like but more metal-like conductivity, thus exhibits much lower voltage and temperature dependencies. Metal resistors, of course, show still much lower VT dependencies.