Sizing problem in HFSS

Hi everybody
I have some confusing problem in simulating with HFSS
I do lots of simulation (circle slot on ground with microstrip feed above)and get different results without Knowing which one is actually correct !

First I use LUMP port with different size and get different S11 ! as far as I know lump port only excite one mode (here should be Quasi TEM ) so according to HFSS Full Book (EXP 6.5), size of port shouldn't affect the S11 but actually it did!
any one who know why? and what is the best size of this type of excitation? and how we can get that, (like waveport)?

another confusing things is radiation boundary !
when I use air box that have overlap with wave or lump port ( for microstrip feeding) I got different S11 from when I use internally excitation in the radaiation box (I use a PEC in the back of waveport)
Here I sow both of them in uploaded file of Hfss, which one is exactly correct?

Actually my English isnot so good so I hope you could understand my problems.
anyway I will be very tankful for any help regarding my problems!