Well as ur images depict, ur antenna impedance seem to be near to 6 ohms and so U have a good return loss, the other case that u have taken of 50 ohms, u dont have proper returnloss as there is a mismatch. As for ur point of why the returnloss is same for any other value other than 6ohm, A Miss match is a miss match whether it is by 50ohm or 100, The return loss cannot be same if ur port impedance is 50 and 100, they both will be different atleast slightly. If u plot them with the case of matched one, then u surely wont be able to say the diff, but plot them separately U should be able to see that there is a slight diff in the level of return loss for different mismatches..!!