I get it, I just say :
THE INCREASE IS LINEAR IN TERM OF FLAT DPS (DAMAGE PER SECOND) and NOT LINEAR IN TERM OF %DPS. Stop forgetting to say EXACTLY what you're cheking. If your DPs is 250 and after some changes in your stuff, your new DPs is 275, you can say about your DPS "the increase is 10%" or "the increase is 25", both are correct. When you are testing/calculating your DPS, you are just checking the final outsome and not the increase in %. You are taking the highest DPS as the BiS.