Hi Geokhan, from what I understand you are asking why moisture content and % bitumen use different formula's. The first thing that comes to mind is apples and oranges, soil and asphalt. The second thing is that moisture content used in soil mechanics is no the same as % water and % solids used in pipelines. I believe there are other threads which discuss the difference in what is the water percentage based on which profession you ask. To get to the heart of why things are calculated the way they are it deals with the research in the field of study, soil mechanics and asphalt, which are not the same field of study.