(An explanation of the 2x2 section barrels layout and a list of the items available in each item set can be found at the barrels page on TheKolWiki.)
The upper left cell means that there's a 44.4% chance that the upper left 2x2 region contains top-row quality drinks (), 33.3% chance for restoratives () and 22.2% for stat boosters ().
If you choose a barrel in that section and determine that it's e.g. a restorative for you today, then click on the restorative icon in the cell to update the probabilities. At rollover the barrels are reset and a random layout is chosen again.)