go to adwords - dump all your keywords into an "all time" keyword report.
look at it - does the regular interface (on screen) figures match what the xls says?
not for me. how about you?
not only that, but the same report a month ago does not match the data from today - by a rather startling degree.
i see gaps at about 20% or more depending on how you count it.
one keyword converted once in one report, and 10 times in yet another.