How Many Years, Actually?

Pos Malaysia released a set of three stamps on July 7, 2011 to commemorate 100 Years of Aviation in Malaysia (click here to see the stamps). So, in year 2011 Malaysian aviation is celebrating 100 years. But, hold on.... in 1997, Pos Malaysia also issued a set of three stamps entitled 50 Years of Aviation in Malaysia (see the stamp image below). The gap between 2011 and 1997 is just 14 years.... but the time gap between 50th anniversary and 100th anniversary is 50 years.... Now, I'm really really confused.....