It is a translation and the old testament was written in the midst of a whole 'nother culture. There is no error here, I am sure of that.
Consider, for example, Japanese. If you ask a Japanese person, such as my daughter-in-law, "Yuki, do you want more coffee?" It is not unusual for them to respond (particularly if speaking in Japanese in a formal situation) for them to say something like, "No, coffee exists."
While that sounds like it does not answer the question, it does. In their culture, speaking about oneself is often seen as rude, so they eliminate reference to themselves and simply indicate that they need no more coffee by saying that "it exists"....
To us, it sounds nutty and odd. But to them, it is proper and respectful. When you read the old testament, you are reading things written in other, old languages, from old cultures. With the drive to change little and translate accurately, I can see how such oddities enter the Bible. I am confident that, in the original text, a person knowledgeable of the language would have no issue at all reading the verses.
What does the verse really say? It says that David stood on God's promises in great faith and bravely faced the giant. It says that David was victorious. And it says it all thru a translation, and thru centuries of cultural and intellectual change.