Well, for one thing, a full sized grave most likely wasn't dug for the burial of this body. They were working on a garden, not digging an inground pool or something when they discovered the body. So, I think it's safe to assume that the hole originally dug wasn't anywhere near 6 feet deep.

In the 70s who knows what happened? A contractor or other person related to the project could have been living in the house at the time.. it could have been abandoned, or empty.. the residents at the time could have been having work done and the contractors buried the body when they weren't looking.. the resident could have been paid off, it could have been done at night or when people were on vacation, etc. etc. etc.