Some things that most financial authors acknowledge is that FICO continually upgrades its algorithm for determining your credit score, they are really good at it, and they are continuing to get better.
The article mentioned by Chuck says that it was last updated in 4/2013. Most assuredly FICO has tweaked its algorithm a few times since the article was last updated.
From observation it seems that when an author informs the public of a method to game their FICO score, the engineers take steps to prevent inaccurate scores. An example of this is to keep your accounts open after they are paid off. For myself, I found that behavior is not a necessary part of achieving or maintaining a high credit score.
The bottom line is that short term fluctuations should not concern you. By paying your bills you will get a good credit score. A credit score does not in and of itself indicate financial success, rather a balance sheet does.
My advice would be to do a budget (seems like you have that done well), pay your bills (ditto), and focus on how you can get more into investments (improve here). The credit score will take care of itself.