Can't really say much besides "glitch". Sometimes vertical alignment just slips a pixel or two out of whack. Scroll up & down and it fixes itself. It isn't computer- or version-specific. Not necessarily in tables; it can happen any time. But the vertical lines in tables are of course especially obvious.
(To answer the inevitable "Why..." question: Back around 2003, Camino was the only browser that had a popup blocker override. Safari had only got as far as the blocker itself-- That Other Browser of course didn't even have that-- and Firefox was still in development.)