I can’t speak for everyone, but I’ve discussed this with others who have the same issue. For myself, and those who have the same experience (with whom I have spoken), there is a small dimple at the very base of the penis. For whatever reason, this dimpled area seems to have tighter skin, which causes not only shorter measurements, but can hurt pretty damn bad, should one try and press hard enough to get a full BP length. So, my friend, fear not, for you are not alone. I measure slightly off-center, having the eduge of the ruler being as close to the dimple as possible, without actually being hindered by the tighter skin.
"Only enemies speak the truth; friends and lovers lie endlessly, caught in the web of duty". -Roland, in Stephen King's The Last Gunslinger