This is really optional essay material, and don't be put off.
I am assuming you got a 2:i or a 2:ii (UK speak - you are saying first, so I expect similar). First's are pretty rare (top 10%) and the rest fall into 2:i and 2:ii. I got a first, and my GPA looked decidedly dodgy when I worked it out.
Really, if you are concerned, explain it in the optional essay. I did in my case, explaining why this was the way my results were, the explanation and reasons why (not a sob-story, simple unemotional fact) and how the degree was structured.
I don't know whether it made a difference - I got admitted at a school I really wanted, so I can only guess it was the right move. Maybe they didn't care for it, but in your case it sounds like it will make you feel better and is likely to help.