Thanks for sharing the test results. I assume that during the test the samples were stirred or the probes were agitated. I tend to see drifts too, but only when I don't move the probe. Also, were the samples well buffered? I think the lab grade pH meter really shine when the sample's pH is very weakly buffered. Soft water is a good example. But most solutions we test in brewing are strongly buffered.
Keep in mind that the simple Hanna meters are advertised as replacements for strips. They seem to have a comparable accuracy. Given the precision with which we want to hit the mash pH, 5.3 - 5.6 is good, they should be acceptable. But if you are technically inclined you may look for more accuracy in the future. It is my opinion that a good simple pH meter with buffers and storage solution can be had for under $100. That is the SM101 that I mention in the article.