[Antennas] Elementary Antenna Question

[WX5L]

Hello

I was thinking about antenna efficiency and was sweeping my quad with my
MFJ-259 and was curious with the findings. 

Basically why does the antenna resistance(50 ohms) does not correspond with
the SWR dip?

For example in my case........I have a 50 ohm reading with the MFJ at 14.025
with a SWR of 1.5 to 1 but the SWR dips to 1.2 to 1 at 38 ohms and at
14.325.

I would think the antenna would be most efficient at a 50 ohms reading
rather than 38 ohms even with a lower swr. 

I am not using any baluns but do use 1/4 wave matching transformers of
RG-11.

Seems picky I know but if someone has an answer I'd like to know.

Thanks,
Randy
WX5L