How do I politely say "DUH"?

I just was given the opportunity to take a class/training, through work, on Introduction to Analytical Skills. It was an interesting class for sure. During much of the class, I kept thinking that I knew a few people who might benefit from a class that tries to encourage thinking outside the box.

But today, I get a task from a Supervisor in our unit noting that an item hadn't been properly updated.

OK, here's the thing...I'm no genius and no one is perfect, but I am good at my job...dare I say, GREAT. I can say this with confidence, because everyone I work with compliments me. I take pride in my work. So to see that something wasn't done was shocking. At first I questioned myself, wondering what could have gone wrong. Perhaps I entered the wrong date?

Then I go into the system. I look, and sure enough, it's not updated for the date in question. Weird!!! But then I look again at the original paperwork. The dates originally requested were February 1, 2012 through April 1, 2012. She highlighted that April 1, 2012 through April 30, 2012, noting that they were not also updated. Then my eye catches it. Um, DUH. I can't updated "April" because the original request only goes through April 1st, not through the entire month. So, no, it did not get updated, because that is not the way the paperwork is set up.

So now I find myself having to sit back and reflect on a) Why do these people make 3x more money than me, but can't figure out this work?; and b) How do I nicely say "DUH" to a Supervisor?

I shall defer to MY Supervisor for direction. LOL. Yes, that was a work rant. It's irrelevant and changes nothing in life. It doesn't even actually affect me AT ALL. Yet, I felt the need to vent. Thanks for listening my friends!