I paid $25 on one card, which decreased the balance to $0. I immediately lost 13 points on my Equifax report from paying this $25 balance according to MyFICO.
Why? I use the card regularly, but I always pay it off in full. I would have thought paying it in full each cycle would improve my credit score, not hurt it.
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.
If you want to see the nitty gritty of how debt is calculated into your FICO score see http://www.creditcards.com/credit-card-news/fico-credit-score-account-amounts-owed-1270.php
I couldn't find a specific reason why it would have dropped for paying this bill but there are many different factors that go into the FICO score.
This quote is helpful: "A high FICO score can best be achieved by regularly and responsibly utilizing a few accounts of different types, while always paying on time, keeping balances low, and applying for new credit only when needed."
But as a general rule monitoring your credit score for minute changes is not really neccesary. As long as you generally follow proper guidelines (pay your cards when they are do, keep your cards for a longer amount of time, don't max out your cards, etc...) then you shouldn't need to worry about the small changes and over time your credit score will rise.
