The maximum percent of yield should be 100%, so It wasn't possible to make a higher than 100% yield like 105% in this problem. If that happens, it will contradict the law of conservation of energy as you can't create mass.
The increased mass might be produced by impurities. One of the examples would be if you rinse the container and left some water. The water is not calculated as a substrate but will increase the results weight.
There also possibilities that some gas in the air react and produce a solid/liquid material. If you don't weight the gas it would seem that the mass increased.
