For DBMM, the weighting of the competitions is more of an art than a science. I think only the UK ones get variable weightings. The original idea was that there would be 2 at 120 points, 2 at 100 points and the rest at 80 points. Some are protected, so your score always counts. Of the unprotected ones, the best score are chosen. In total only 6 scores are chosen, summed and the result divided by 6 to get your ranking. If you do less than 6 comps, your score therefore suffers. If you do very badly in a protected comp, your score will suffer - the idea being that the 4 protected comps are the ones that most people go to so if you crash and burn in those it will tell.
On the website, included scores are green, discarded are red. If you hover over a score, you will see which comp it came from.