It is a good question. I am not familiar with the specific test method the Mebak test methods list both the spectrographic test method (ASBC) and the HPLC method, so without a specific reference to the test method used, it is really meaningless to compare. My guess is that you are looking at the comparison of the HPLC (High Pressure Liquid Chromatography) versus a spectrographic method (UV measured at 275 nm) which is most commonly used due to the lesser cost of the equipment and test process. I think I got the same lot of Hallertauer Mittelfueh marked at 2.8% AA. Since they come from Germany, the testing originally may have been with the HPLC (EBC method), which explains the difference.
My take on it is to use the number you in which have the most confidence. Since the spectrographic method is the most commonly used, most of the hops you buy will be measured using this test. Additionally, the IBU models we currently use are based upon testing performed on a spectrophotometer (UV/Vis).
This may be changing as the brewing community and craft brewers in particular are more interested in the specific hop oil breakdown which you can only get via HPLC. But that will not change the models unless someone bothers to spend the time to redo all the testing needed to produce a modern model based upon modern hopping methods and HPLC analysis of both the hops and the resultant beer. I have access to both instruments, but actually prefer my taste buds since the UV method is, IMHO, misleading given modern hopping methods as a direct predictor of bitterness. I have no real comparison to the HPLC method and my bitterness perception.