Why do you need to know "why"?
Isn't it enough that two or three guys tell you the Remington ammo is problematic in a luger?
Any answer would be just more speculation, it could be too short, or too long, or the charge too small, or the wrong powder type- it really is irrelevant! JMHO
I have had success with S&B, Blazer brass, Olympic(Greek), Winchester white box, Magtech, Fiocchi, some reloads, and Wolf steel case -
most anything EXCEPT Remington.