Not all theses are groundbreaking. This is true in math as in any other discipline. Oftentimes, an adviser has a hunch that going in a certain direction might yield something interesting, but that turns out to be wrong -- sometimes because the student didn't work very hard, or simply because there is nothing there. For example, in math, the original conjecture might simply have been wrong; it may have been correct but too hard for even a good student to prove; or, maybe most frequently, it is true in some cases for which it was proven but these cases end up looking rather insignificant.
So, from the perspective of a department, what do you do? The student did work, his work ethic was average, nothing of great significance came of it at the end of 5 years, but it wasn't for lack of trying. Some incremental progress was made, but nothing that looks particularly impressive. Do you kick the student out of the program after 5 years? Do you let them work on this for another 1 or 2 years with uncertain prospect? Or do you simply declare victory, bury the topic, and make sure the student graduates in a reasonable time and everyone moves on with their lives?
You will find that this last option happens surprisingly often. The number of real breakthroughs is, after all, rather small.