Item Frame Fence Placement

It a block has a fence under it, and a fence beside that one, you can't place an item frame on the side of the block with the fence under it. You can place it if you remove the fence under the block, which makes no sense. This is hard to explain, so please watch the attached video. (File size issues, here's a Youtube link: https://youtu.be/3ba8V-5nxFY )