Looks like his last measurement had the highest Tc. Assuming that his experimental setup is not at fault, I'm wondering if hysteresis and/or a "learning" effect is occurring, and how the duration and transient characteristics of the applied voltage affects the results. Wouldn't it be cool if Tc went up with use?
I am also more prone to see this as a measurement error or sample modification between the various experiments.
In his own words:
"RESEARCH NOTE: Copper-oxides have been found to be strongly hygroscopic. All tests should be performed immediately after annealing."