Fairly often when working in Excel I start getting the pictured problem. I left-click in a single cell but it selects several cells, always down, most normally 3 cells. This particular example selected about 10 cells. This does not happen in the other sheets in the affected workbook, not in other workbooks that happen to be open at the same time.
My only recourse is to close the workbook and reopen it. Nothing I've tried allows me to correct the problem with the workbook still open. Rarely, I have to open and close it a couple of times to get the problem to stop.
Win7 x64, Office 2010 Pro Plus SP1. Dell Vostro 470 8GB RAM.
(Although it was also prevalent in Win7 x32, Office Pro Plus, 3 GB RAM, and a lesser machine.)