Test sails are not all the same. A good test sail is hard to get and few magazines have reliable ones. Voile and voiliers is one exception, Yacht.de is another. Both say how the boat was equipped, the test sail conditions and give exact figures with all elements regarding speed and how it was achieved including sea conditions.
I prefer comparative tests with both boats on the water and even if those two magazines do them they are not the more frequent. The Oceanis 38 was tested in the water with the Varianta 37 with light winds, being the Varianta noticeably fast and I am sure that the Varianta had mo racing sails but cheap ones (you can see on the pictures).
I sail tested once a Luffe 4004. That was a windy day (about 20K) and the boat had non reefing racing sails. Oluf was sailing the boat and with the main alone we were soon doing +8k. Those narrow boats have little drag and can easily get to hull speed, even a bit over it. remember that they were sailing the 36.6 with a spinnaker when they reach 9K with 17K wind. A more light beamy boat like the Pogo 10.50 would probably reach the 9K with considerably less wind or with that wind that speed without a spinnaker. But if you loaded it too much the lose in speed would probably be more noticeable than on the Luffe.
The Oceanis looks like a Pogo...except in the weight, RM and sail area. off course, downwind speed too.
I agree with you regarding the boats being unloaded and with very little tankage aboard. A real cruising boat (except Eric's one
) will have most of the time a thousand kg more and not all boats are affected the same way by an heavy load.
Regarding Calibrated instruments that can have some effect but testers are not dumb and for sure they have a look at both instruments: GPS speed and log speed and that limit margin error.