Pumps connected in Parallel and Series PROBLEM

Dec 2013

Recently, we conduited an experiment from Fluid Mechanics Laboratory course, and, while doing my report, i was angry that results are wrong. Then i started to do trial and check in my excel and... results got preety similar to characteristic curve that is seen on manufacturer's website! Even though - i do not understand why (M1+M3-M4) gives correct result. Here's some screenshots!

Please, help me understand this!