It's not designed in, rather that it can't be designed out.
Prewar cars had ladder chassis - so called because they look similar to a ladder. A two dimensional chassis can't be braced three dimensionally, so it will twist laterally. If it's any consolation, this also happens with 60's monocoques such as MGB and Midget, where the door fit deteriorates if jacked up on one corner. Try jacking one wheel of your Mog, and you'll find that the doors won't close quite as well. The twist is only slight, so the floorboards are Ok - but they do have a flexible sealant applied.
