quadcopter motor esc problem

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Hello, I am trying to build a quadcopter ,I got my 4 motors and 4 esc's ,even though its not conventional I tried checking their speeds o found out that for the very same ESC and pulse the speeds are different for the 4 motors ,3 of them are almost near but one motor have a difference of 100rpm, also sometimes the speeds are notably reducing (much greater than for effect of voltage drop on battary) however the lipo i'am using is of good quality,....i am new to this field any help ?