Same Sketch but Different Result

Sketch is same as in the video but giving unwanted region to me (Sketch18). What is the reason?

I was able to get rid of this problem by sketching on the origin plane. But I wish to know why above way was failed in my case?

SolidWorks Tutorial #268 : advanced screwdriver bit (PZ2) - YouTube

Time 5.08

SolidworksSketching