Seven in a P 64

[ourbigbluehouse]

I had a strange happening recently and want to see if anyone can tell me why. Recently after a visit to the range ( inside in NE Ohio weather - no need for windage ), I used a speed loader to load the mags. On one of them I managed to accidently load seven and not six. It wouldn't load into the pistol until I took one round out. My stupid question is why does this happen (other than my stupidity)? Please don't tell me they designed it that way either. If the mag will hold it, why didn't it click into position?