The lack of ink on the CM's stamp pad might be a valid point. Some days when I have been hand stamped it felt like someone just squirted hand sanitizer on the back of my hand. Other times I have barely felt anything.
This is only true for 3 or more day parkhoppers, not just a normal single day ticket.
This would make sense, since the whole purpose of the picture taking is to prevent the use tickets by multiple individuals, and the sales of unused portion of multi-day tickets. Probably not a lot of 2 day tickets be sold for the unused portion.