The measuring tape end needs to be directly above top/base of penis extended to tip, I guess he felt you were displacing the tape somehow to make your cock appear larger, which is easy to do...just by a slight repositioning of the tape. I've experimented with measuring too, and notice variations can occur. There is also a 'bone pressed' measurement and a normal non pressed measurement. The bone pressed is a universal measurement for all, since some have more of a fat pad on pubic bone.

Your other measuring pic looks better as far as tape measurement placement.