Decrease completion rate day by day

[gr_nisan]

I cancel an order before 15 days then I saw my completion rate was 83%.

After one day later I saw, completion rate 80%. after 2 days later I saw 75%.

then I got an order and completed. When I completed an order then I see order completion rate increase 80%. Wow that nice.

But Today I see the order completion rate is 75%. 

I don't know, why up and down order completion rate ?

Experience man give me suggestion.