in the middle of ,“在…的中间/中点/中部/中央”等,是强调的“点”的概念；
英文解释：point or position, which is at an equal distance from two or more points, etc;
point between the beginning and the end.
而in the middle part of 强调的是部分的概念,即是一个范畴或者说范围问题.