Yes, I've experienced this myself.
When I went on a cutting diet from February 15 to May 1, I didn't cut back enough on calories. So I had maintained my overall weight, but still lost about 5% body fat (18% down to 13%). This would indicate an increase of 9 lbs LBM during the same period, substantiated by an increase in size both on the biceps and calves.
That said, I should probably mention I'm still pretty much a newbie, having only just reached the one year of training mark.
Though, yes, the scenario you present is very likely to occur in any trainer.