As
@rianne stated, bells are recalculated when you edit posts. Here is an example:
As you can see from the post number, all three of these transactions concern the same post.
1. This transaction showing that I earned 11 bells is the amount of bells I originally earned from making the post.
2. This transaction showing that I spent 11 bells shows that the bells were removed when I edited the post.
3. This transaction showing that I earned 1 bell shows the amount of bells that post is now worth after editing. I removed several paragraphs, hence the post now being worth significantly less than before.