Question on parametric hfss

Hi all,

i sweeping my lump port cap in my antenna design..however i got different results each time i sweep.. any one can explain the use of parametric in hfss?

Here's my setup,
parametric setup -> linear step--
start = 1pF
Stop = 5pF
Step = 0.1pF

The S11 plot is diff from this setup

start = 1pF
Stop = 2pF
Step = 0.1pF

? why?