Thank you- I often forget that stuff I just know because of work, isn't common knowledge. With that in mind, it occurs to me that something else that it may be useful for me to share, is a possible functional reason for both graves being so shallow (beyond that of speed or difficulty with hand digging).
When compacted soil is dug out, the soil is broken up. Try digging a hole in a previously un-planted and undisturbed area of your backyard, and then fill it in again. The soil won't all fit back in because air pockets have been introduced. It takes time and weather to settle the soil back to being compacted. How long depends on the type of soil ( sandy soil settles faster than clay, but we are still talking many months even here in the UK where it rains often).
If something else of a reasonable volume has been placed in the hole before the soil is replaced, then the soil displacement is even more marked.
This is known as a "spoil heap" and the larger and deeper a grave is, the bigger the spoil heap will be. Municipal cemeteries will use spoil heap soil to top up graves that are sinking due to settling and breakdown of buried coffins. But in smaller burial grounds the gravedigger will be paid to "cart away" the soil from site, so that it is not an unsightly heap that is immediately visible to visitors.
Thus, a shallow grave, dug no bigger than absolutely required, and reinforced with wood and stones to reduce obvious sinking, makes sense for an illicit grave. The spoil heap would be small, and easy to disperse in small amounts around bare earth areas of a property.