Different result after using HoughCircle?

I have a video x. Case 1: Get a frame from the above video. Change frame (color image) to gray image. Then using medianblur. Call HoughCircle and I receive list of circles. Have 3 circles. Case 2: i save the above frame to jpeg image. Read this image and apply the above functions. I also receive list of circles. But different result about Quantity. I just receive 2 circles. Why? thanks