General practice, not a claim about any one account.
DifferenceCaching and page speed plugins
Caching can serve an old copy of a page. Tracking code may then run less often than the page loads, and traffic can look lower than it really was. Check the caching setup before you compare site analytics with other sources.
DifferenceDuplicate tracking and form conversions
Two plugins that both add tracking code can count the same visit or sale twice, or one can block the other. A form conversion may be counted in one tool and not in another, depending on how the form sends its data.
DifferenceTime zone and scheduled publishing
A post can be scheduled in one time zone and shown in another. Publish dates in the site, in analytics and in the ad reports can then point to different days. Use one time zone for dates in every report.