There are different items to consider:
A/ Your torrent programme updates our tracker each 30 minutes about up- and download. If you don't STOP a torrent before closing the torrent programme is closed, you will loose the stats since the last update. Note each torrent runs on it's own schedule. In Vuze/Azureus you can see that schedule counter in the "Details" view, in µTorrent/BitTorren in the "Tracker" tap on the bottom of the main window
B/ The UL and DL below the blue band shows the accumulated statistics of all these updates for all your torrents since the account was created.
C/ The UL and DL in your Profile's torrent list or a torrents peer list shows only the stats since your actual start of the torrent., e.g. lets say today 15h00 until now, minus missing since last tracker update (it is what is meant with session based). This compares to your torrent programme, which accumulates per torrent all stats since you put it into it, e.g. yesterdays and before days activity before you stopped shutting down your torrent programme or PC, too.
D/ Note there is a short delay between tracker update and it get's visible on the site. In addition once you reach important absolute UL and DL amounts only long time periods will give a visible change because of the resolution only large UL and DL changes are visible, e.g. if expressed in TB you need a change > 10 GB to see a change in the last digit.
Hoping this clarifies a little.
Uwe