Hi Laura,
I believe when you add the pageviews metric in a report it does not display the total of pageviews for the site but the total of pageviews for the report.
I believe this documentation will complete what Akhil provided in his answer : https://microsite.omniture.com/t2/help/en_US/sc/implement/index.html#channel
Adobe recommends populating the channel variable on every page and the channel value do not persist. If you take the following scenario :
Page 1 and channel set
Page2
Page 3
Page 4 and channel set
Per the scenario I will have a total of 4 pageviews for the site as there were 4 page viewed (and for page report). However i will only have 2 pageviews for channel (Site Section) as it was only set in 2 pages.
So you have less pageviews in your site section report (channel) than in the pageview report or the pages report because the s.channel variable is not set on every single pages.
Best regards.
Alexis Cazes