In the context of scientific research these differences may be crucial and should not be taken lightly. They could be the explanation for the differing results and should be taken in account for when designing new research before the conclusion "case is closed" could be made.
It must be of uttermost interest to investigate the differences in test context for theses positive tests and tests not being positive. Note that there are no tests showing it's negative to use SC!
1. Can the results be dismissed because these tests were made in South America?
No, climate may influence, but no research verifies any hypothesis that all bees in the tropics are resistant to the Varroa mite. Not resistant bees are kept in similar climate.
2. Can the results be dismissed because the bees in these tests were Africanized?
No, There is no research showing any such differences between African and European honeybees. On the contrary these types of bees are both Apis Mellifera, closely related and interbreed easily.
3. Bees in other tests have not been resistant to Varroa.
This is an interesting difference. Today there are reports of quite some beekeepers in different places not treating at all for Varroa mites and bees seem to tolerate the presence of mites and still produce normally. There are SC and other bees among those. There are tests showing most probable reasons for the resistance are changes in the behaviour of the bees. Could these changes in behaviour in some way have effects on the bees' relation to an SC environment?
4. When bees are born in SC they may well have been fed differently, amount and composition of food.
Nursebees born in SC may well be feeding larvae differently. Differences in feeding behaviour have been shown in SC-colonies. Different feeding has influence on the phenotype of bees. How big behaviour differences are resulting? In the Brazil tests the nurse bees have been born in SC and fed by SC-bees. In most of the other tests the nurse bees have been born in LC and fed by LC bees.
5. There is a growing awareness of the influence of the exchange of bees between bee colonies.
It can be quite big under certain circumstances. Two of the causes are drifting and robbing. Robbing can be slow, almost not noticeable, and it can be intensive. This exchange of bees can be destructive for the possibilities to get an accurate test result. Even 1.5 km can be too short a distance to avoid significant not wanted influence.
6. When bees are treated with something that kills parasites or pathogens, they are also loosing beneficial microbes.
This can influence their immune system and their behavior. In the short perspective of course it's understandable if a beekeeper wants to save his bees from dying.
7. Wax in combs has been found to contain a substantial amount of different chemical residues.
Residues from varroa treatments, AFB treatments and plant protection sprays. That of course applies for such environments where such a result is possible, which applies to many areas in North America and Europe. Chemicals in sublethal doses are known to influence immune system and bee behavior in a negative way.
8. Heritable changes take place in the genom of all living beings, also bees, depending on changes in the environment.
It's been more and more discussed among genetics. It's called epigenetics as the changes is not taking place in the composition of the DNA in the chromosomes, but in how the genes and even fractions of genes are expressed, "turning them on' or 'turning them off'. This effects the production of proteins and thus the phenotype of the bee.