Insert large image isn't the size set in media settings

i’ve set the large image size in my media settings to : 642px to fit very nicely in my theme. i upload an image. i go to insert said image and the large image size is 580px. wth? i’ve set the max-height to 1000px and the image is a landscape image so it isn’t hitting the max-height barrier first. this is extremely annoying. anybody know what is up?