One of my favorite movies is The Princess Bride. In the movie Wesley out wits Vizzini by building up an immunity to Iocane powder, by taking small amounts of the poison over time. This is a similar theory to what is happening to your weeds when you skimp on your mixing rates, and apply too light of a rate of herbicide, or running water right after, diluting the solution. Every time this happens the weed grows in resistance to the herbicide, and will soon be able to outwit your attempts to kill it, just like Wesley