Red metal-topped bedrock is slightly taller than other blocks
The red metal-frame-topped bedrock blocks are very, very slightly taller than all other block types. As a result, the block arrangement in the screenshot above is impassable on the left, but passable on the right. Bolt traps can fire through the triangular hole in either case, but in the case of the left arrangement the player cannot get to the back of the area.
This also applies to other bedrock-based decorative floor blocks, such as this impassable arrangement, where it's impossible to reach the trap at the back: