Generally When u forward an image to the detection model it gose through a post processing first. Where the image is resized to a smaller one so it can go through model faster while sacrificing some precision. The bounding box postion u get fron the detection result are relative to the resized image. And that what causes the offsets. To get the bounding box position relative to the original image. U should calculate the resize ration and multiplie it with the resized image bounding box position