tnt, when gets ignited, doesn't stand still on a the fence

When I light a tnt, the tnt falls through the fence. Interestingly, it doesn't fall through iron bars.

What I expected to happen was:
the tnt stays on the fence.

What actually happened was:

The tnt fell through the fence and the two textures merged.

Steps to Reproduce:
1. Put a tnt on a fence

2. Ignite it

3. The tnt falls through the fence, causing the two texture to merge.