826,
My first thought is that the feed line has become 'part' of the
antenna and has changed the antenna's characteristics accordingly. My
second though, if that 6 foot of feed line was connected directly to
the antenna when measurements were taken, is that you being that close
to the antenna changed it's characteristics.
So, what to do? How about doing that checking from the end of the
feed line that will normally be used? Make any antenna adjustments
accordingly. Don't want that feed line to be 'part' of the antenna?
Well, how about using a feed line to do the checking that's of an
appropriate length to sort of 'cancel' it's self out, to do the
checking?
Doing all that checking will also give you some idea as to the
accuracy of your measuring device, and the characteristic values of
the feed lines, etc, sort of.
That's the 'long' version. The 'short' version is that you're
probably doing something wrong, re-do it.
- 'Doc

Not much of an answer, huh?