Do you have more data? You have two data points and they are fairly different numbers (66.7 and 64.6). Oh, and if you add stiction additives it will affect the numbers - reduces viscosity. I have very little motivation to react to any single data point. Trends on the other hand are very important to pay attention to.
Fuel dilution is another thing to watch. I see you didn't have any, and that is good. None of my data includes samples from oil with fuel dilution. Another thing that can affect the oil viscosity somewhat is driving habits and being heavily tuned. Heavy on the accelerator and the HPOP (and injectors) works even harder for a given mile of driving.
My experience with these labs is that every-once-in-a-while you get a number that isn't very accurate. I have seen this multiple times when I submit the same oil (split sample) to multiple labs. Once Blackstone got a viscosity on 72.1 and another lab (Dyson Analysis) got 69.5. The 69.5 fell right into the expected range. For the price we pay for oil analyses, we are not going to always get the absolute correct answer. Confirmation of results (from split samples, multiple labs. etc) is rarely done due to expense, but sometimes it is an eye-opener.
I found some older/limited data on Rotella T6. I will post it since it was on the OP's "list of 3". Note another data point that I believe to be an error from Blackstone, but unfortunately I did not get a repeat or "split" analysis on this oil change. This one is low vs the earlier example I gave that appeared falsely high.
I have a 2003 6.0 purchased new, in spring of 03. Been running Rotella T6 since it was new. I did add a bypass oil filter, oilguard, one of the "ball of twine types" filter. It gets a lot of sludge out. This truck has basically been a large family car, not used heavily. I'm just shy of 190,000 miles and no problems.