In this case, though, I'm not at all surprised that they both behave the same, because the date format you were using had the dual undesirable properties of (a) looking like a standard format, and (b) not actually being one. Therefore, it would likely be confusing to any code that is trying to decipher it.
Ultimately, by putting "20" in front of each date (great simple solution by the way), you caused it to fail to match any of the date formats (weird but true I think) and so it's correctly being sorted as text.
I am not particularly happy with the way the format detection (especially for dates) is handled in the tablesorter code right now, and would love to redo it, but it's gonna take a lot of testing to make sure they're working properly, and I just don't have the time right now. Glad you were able to make it work for you!