Depending on how you calculate efficiency, your volume measurements can also have a huge impact. For example, if you expect to make 5 gallons at an original gravity of 1.060 but you boil too hard and only ended up with 4.5 gallons, then your gravity will look way high by a ratio of 5/4.5, so you might get 1.067, which makes it look like you got like an extra 10% efficiency, when in fact your efficiency is probably the same as it always was, you just boiled it down too hard. It can happen if you don't account for volumes in your measurements. Think about it.