Featured Image not cropping to size specified

I’m going absolutely mad trying to figure out why my “featured image” for a post isn’t cropping to the correct size. It insists on coming up 1 pixel short each time. Here’s what I’ve got.

An image in my template that needs to be 310X215. I have this set as the thumbnail size in the Media Settings and I have the “crop thumbnail to the exact dimensions” box checked.

I go to my post and click Featured Image. I select a file from my desktop that is 686X490 in this case. It opens and I use the edit button to crop it to 310X215. I click the crop icon to crop it and when it gets me back to the previous screen, the crop is shown, but the size is 310X214 instead of 310X215.

I try deleting it and working with it again with no luck. I my custom theme’s functions.php, I have specified:

// This theme uses post thumbnails
if ( function_exists( ‘add_theme_support’ ) ) {
add_theme_support( ‘post-thumbnails’ );
set_post_thumbnail_size( 310, 215 );
add_image_size( ‘post-thumbnail’, 310, 215, true );
}

Can anyone give me any idea why this crop tool isn’t working properly? Is it a bug with the actual crop tool or is there some other setting someplace else that’s decided it would rather my images be 1 pixel shorter than my template requires? It’s absolutely driving me crazy. Thanks for any advice.