Well it seems that quite a few GA users have observed unaccounted-for behavior, particularly during the past couple of months.
For instance,
18 - 19 May 2010:
more than 40 different GA users posted to the GA
User Forum all regarding the same
issue: no data whatever was
recorded in their GA Accounts
during the 18th and 19th of May. No
response from Google and nothing in
the GA Blog. Several users who had
other GA accounts that were functioning normally during this period, suggested that the problem might be caused by recent changes by Google to the GATC (which was in fact recently revised)--many of those who posted on the Forum said that indeed they had recently added the latest version of the GATC to their Sites/Pages.
6 - 9 May 2010:
Over 50 GA users reported, by
posts to the GA Forum, a complete
GA outage during the period 6 - 9 May
(no data appearing in their reports
for at least one of those days). This
time a GA Team member did respond with
a one-line response "there was a
delay in reporting, no data was lost."
This post also referenced a Twitter
message 4 from GA stating the same
thing.
In addition, i've seen a half dozen, perhaps more, recent posts (past 60 days) on the GA Forum in which users reported significant discrepancies between an aggregate figure and the sum of the constituents--both sets of figures from the same Report, e.g.,
Numbers Don't add up on the Absolute Unique Visitor's Report
Search Engine drill-down visitors don't match total
Neither Post was answered (either by the GA Team or anyone else).
Finally, since it's just a matter of clicking a menu and selecting a different option, i suggest comparing the figures you recited in your question with the analogous figures for Page Views, which is probably the simplest measurement in client-side analytics ("Visits" by contrast is strongly influenced by user cookie manipulation).