I don't think you have cause for concern - everyone is differant. I'm not sure how they come up with the idea of an average weight gain since there is no such thing as an average women.
When Yasin was concieved I weighed about 50kg the day before he was born I wieghed 75kg so I had put on half my body wieght!!! By the time that number 2 was concieved I was back down to around 50kg without bothering to diet or workout.
Just so long as you are healthy I think the best option is not to weigh yourself. The only reason I know my weight at the end of my pregnancy is because I was bored and there were scales in the hospital corridor. Other than that time my midwives never weighed me so it was never an issue.