Total Organic Search sessions does not match with secondary dimension
I am comparing organic google traffic from Jan 1, 2017 - March 31, 2017 when compared with the same time period in 2016. With no secondary dimensions selected it says the total google/organic number of sessions in Jan 1, 2017 - March 31, 2017 is 51,343 and the total number of sessions in Jan 1 1, 2016 - March 31, 2016 is 54,191.
I found the difference to be 2,848 sessions.
I apply the Landing Page secondary dimension and started to add up the differences in sessions when broken out by landing page and the number is more than what the original total was. For example,
Landing page 1 saw a difference in 1,333 sessions
Landing page 2 saw a difference in 2,000 sessions
and so on.
I do not understand why the landing page differences don't add up to the total google/organic sessions.