The wort temperature can affect the reading, which results in a higher than normal efficiency. The temperature is supposed to be 60F° for a good reading. Some devices make an accommodation for temperature, but they are not perfect.
Another thing to consider is the amount of grain. If there was an error in measuring the grain, it could increase the OG. Also, if you mash for a longer period of time at low temps of 150° or below, you will increase efficiency. And, too much crystal malts will give you an increased OG reading.
Cheers,