The published detailed data DxO provides is (mostly) useful, their overall score number is not so useful.
In the case of lens scores, they attribute some of the lens' score to the image noise levels produced by the body they're testing it on.. Really.
So, other subtle changes in sensor and signal processing between otherwise similarly spec'd bodies can affect the outcome of their overall score.
I was also surprised at the very poor score of that old Tamron lens, I've seen some lovely images shot with it and I've been thinking of getting one for my collection.
Use DxOMark's overall lens scores with a big pinch of salt. I also suspect they only test one variant of each lens in many cases, so a better or worse than average one can easily show some skewed results.
FWIW, for the past number of years, one of the primary reasons I avoided coming over to the Pentax camp was the poor metrics of many of their lenses.
After the arrival of the K5 series and discovering how much I liked the ergonomics and UI I decided to try the brand anyway. My K52s is now my favorite and most often used camera, a D800e is a close second and an old bargain price used k10d is getting more use than I expected.
What I've found, with the limited shooting I've done so far with Pentax systems, is that the lenses may not always measure up to the best of the competition for sharpness, distortion or CA, but they still produce terrific images. In many cases I find I like the bokeh and sometimes the microcontrast of Pentax glass better than the competition. Every system has its own particular quirks and personality and I find Pentax is a very enjoyable system to use that's capable of providing a competitive level of performance for typical shooting and has only a few areas where competitors may perform better.
But then again, I wouldn't put the K5 in the same league as some of the competitors' more specialized or high-end cameras; in it's class, it's a top contender and so are the lenses.