Penis size measurements are not final. Many things may interfere with the strength of the erection. Men will experience the strength of their erection changes throughout their life, even over a course of a single day. Many things may interfere with the strength of an erection and you should not take the initial measurement result as final. Changes in your lifestyle, diet, and even simple things like removing pubic hair may have an effect on corresponding penis size measurements.
Whilst this is a touchy topic, considering the intimate nature of the subject. It is science and therefore it can be looked upon as completely natural as a question of inheritability of physical characteristics. The nature of genes and our understanding of the processes involved as genes pass from one generation to the next are yet to be fully understood but we already understand the principles.