Well sometimes, the standing leg can slide in front of the kicking leg so much that the kicking leg hits the ball into the standing leg, causing a massive deflection and top spin and dip, which can massively wrong foot and deceive the goalkeeper, which is a bit unfair.
To be honest, I'm the interest of fair play, surely when slips occur on pens like these, clear and obvious slips, just surely at refs discretion call for a retake?
and iirc i think that double kick rule came in after that Henry to Pires pen that they did back in the day, was to stop that **** rather than a players standing leg grazing the ball