I think you are just an overly concerned new snake keeper! But the fact that you are keeping records and noticing stuff like this is actually a very good sign that you are a great snake keeper! It just takes a little while to learn which things to worry about!

As everyone has pretty much said already, as long as the snake is still eating, I wouldn't worry at all about the alternating between eating 1 and 2 mice. Maybe it is at a point in its growth where 1 1/2 mouse would be perfect, so it is balancing things out?