The Site Connections tab would list all the connections to the QC/ALM server. If a user were to have two sessions open to the ALM server logged in as the same user on the same workstation that would show as two connections.
When looking at the Site Analysis tab you are looking a how many licenses are in use. So those same two connections above would show up as one license in use on the Site Analysis graph.
To test I logged in as three users on my lab server James_alm, Kelly_alm, and Alex_alm (for Alex I had two sessions open on the same machine). Notice that there are 4 connections showing.
Now if I go to the Site Analysis tab and look at the sessions/license usage you will see that only 3 licenses have been used (the Alex's second connection did not use a license since it was the same user on the same workstation).
In the Site Analysis the number represents the user sessions that existed no a specific date.
After these screen shots I logged out of the Clients and Site Admin, restarted the ALM service (to clear all the connections from the Site Connections tab), and then logged back into Site Admin to check the Site Analysis tab. The Site Analysis graph still showed 3 licenses had been used so the graph for any given day would be the total number of sessions/licenses that were used.