Why Did Joe Biden NOT Stand at Full Attention for Fallen Service Members?

A lot of attention has been focused on Joe Biden looking at his watch when the fallen service members from Afghanistan were returned to Dover Air Force Base. You can see that scene here but what was missed was the fact that Biden also NEVER stood at attention the whole time other than when he was saluting. Biden, as you can see, either had his hands behind him at parade rest or folded his hands in front of him. Everybody else, including his wife, stood at full attention with their hands and arms properly down by their sides in stark contrast to Biden. Has nobody told him the proper way to stand at attention while honoring fallen service members? Biden has been in public political life for over 50 years so I am sure he has attended many such sad events so why did he NOT seem to know how to properly show respect this time?