Well so does mine, when its growing we dont really know until we measure, and sometimes we think its smaller because when we look at our dick the most its in Penis Enlargement, and in Penis Enlargement its bigger cause we are strecthing, expanding, and making it bigger, so after looking at that for 1 hour (probably the most time I look at my penis) when I look at it again later on it seems small...
Thats it I guess, there are many factors, I also think that alot of us preceive our dicks to be smaller than they actually are, we think were small sometimes, so we look smaller, the negative thoughts effect how we see ourself..