Why 偏 has a meaning of partial? For example, 偏導數 means partial derivative. But my intuition on this character is inclined and remote. How can I derive this meaning from the regular meaning of 偏?

From 説文解字 I got this


  
   (偏) 頗也。頗、頭偏也。引伸爲凡偏之偁。故以頗釋偏。二字雙聲。尙書。無偏無頗。絫言之也。周易翩翩。古文作偏偏。从人。扁聲。芳連切。十二部。
偏's core meaning is 'lean to the side' as in 偏右(lean right) or  偏左 (lean left)

That meaning extended to represent 'away from center' as in 偏僻 (remote/ isolated) and 偏遠 (remote / far away) 

Notice: '偏遠地區' (remote area) is referring 'an area that is far away from the center of a country'. Even it is remote, that area is still 'a part of the whole country. Therefore, 偏 also obtained the meaning of 'a part of' (partial) by the virtue of opposing 全(whole)

The idiom "以偏概全" - "generalize the whole by examine only part of it" illustrated 偏 (partial/ incomplete) is the opposite of 全 (whole/complete)

The "away from center" meaning of 偏 further extended to mean "away from the main stream" , therefore  偏 also contains the meaning of 'non-standard' as in 偏方 (deviant prescription)