Well as I've never done the 2391, I can only go by my own training and experience.
The r1+rn/R1+Rn tests are listed in GN3 as being step 2 in the testing of an RFC.
However this is often not conducted (especially with new builds) in it's entirety with only the r1+rn part being measured to prove continuity.
With any initial verification, 100% of the installation should be tested, and to my mind this is imperetive when the testing is of a third parties' construction especially as it is likey in this case that there are a number of third parties conducting the construction.
It's all very well taking short cuts when testing your own work, but not when testing other's workand especially when there are likely to be many 'other's'.
With normal double sockets, it is usual to only measure R1+R2 at one of the sockets, so it could be the case that the person conducting the dead tests was not aware that these particular sockets were wired individually, or may be that they knew, but did not consider it necessary to test each individual socket.
This suggests that either the person was inexperienced. Perhaps newly qualified or even not qualified at all.
It may be that the testing is being conducted on a fixed price and the managers/supervisors were pushing for the testing to be conducted quickly, perhaps the person was even instructed not to test each socket?
To my mind, each of the tests listed in GN3 should have been performed in order, and the test and inspection should cover 100% of the installation.
However if the tester has been given an instruction not to conduct certain tests, or not to test 100% of the installation, what can they do?
At the end of the day, they are an employee, and must as such consider that they will be sacked if they don't follow their employers' instructions.
