It's a Panzerfaust. It's literally just a mortar that can be shoulder fired. It has a low effective range and requires delicate aiming. That's why they were trained to get within 15 feet and ambush the tank. The Panzerschreck on the other hand is a rocket launcher. It's rockets aren't very stable in flight as they lack stabilizing fins. That's why the SS couldn't hit anything.

*sits in a polished black leather chair* "An interesting question actually, But you have to give the launchers credit for even coming close. To consider to the variables, You have to be slightly far away from the tank due to the ones within firing on you, your rocket shell deflecting, and the explosion damaging you. Another variable is the accuracy of explosive projectiles back then, Many were extremely inaccurate and a straight shot was very lucky to happen. Many other variables apply like stress and such but mainly they still hit the tank a few times to end the fight or Slow it down."

There's a reason Stormtroopers are notorious for missing.

Maybe that's why they're losing the war?