What tells me a lot more is the difference in size between the two, then look at these ratings again.
If you look at it this way; Ubuntu is a huge "organisation" with developers and contributors by the bucket-load and loads of staff on their payroll as to where Mint is created by a hand-full (if that many) guys as an after-hours project while having a day job. (I know, Mint is based on Ubuntu, but it does not even look like Ubuntu anymore)
Now look at that rating again and see how high Mint is compared to other "larger" distro's.
In my book that earns a lot of respect and says a lot about how good the public in general thinks our Distro is.