I wonder whether the PIE word for five in fact meant "fist", in other words, when people counted, they closed their fingers and when they obtained the closed fist, it was "five"?
Similarly to how the word for right hand gave the PIE word for ten, decm (from dec- "right-hand-side", decsnea̯ "right hand"). In other words, people started to count from their left hand and when they completed the right hand as well, they would have ten.