APFS > Compressed Images Inconsistent

I have backed up four computers this way:

1) Enter Recovery Mode > Disk Utility

2) Run First Aid on all disks involved

3) Eject My Disk (APFS, no encryption)

> APPLE SSD

> Container

> My Disk

4) Select APPLE SSD > File > New Image > Image from APPLE SSD > Save "APPLE SSD.dmg" to External SSD

5) Everything goes fine, no errors, exit Recovery Mode.

On two of the computers this process takes about an hour and the resulting dmg is correct: compression is about 15>20% and the images mount, open and compare fine.

Issue: The other two computers take about 15 minutes and the DMG is not correct: compression is about 90% (dmg is too small to be correct) and mounting attempts return the generic "no mountable file system".

Searched all over and can find no clues, tried PRAM resets, disk checks, everything I can think of and the same process consistently results. All four computers are similar vintage (MBPR) running similar software.

Why do two work fine and the other two not image properly?