Ok, I think I understand. The amp measurements were made with the higher pressure but your normal pressure is 20 PSI. Correct? Also, 20 PSI is with a clean filter and one suction port closed, correct?

But I don't believe it is possible for that pump run on low speed at 10 PSI. Max head for that pump on low speed is 22' and 10 PSI would put it well over that including gauge height. Also, if the filter was reading 27 PSI on high speed, it should have read somewhere close to 5 PSI on low speed. So either there is a problem with the pressure gauge or you did not read it correctly.