How come some PCI video cards have 3 slots and some have 2?

How come some PCI video cards have 2 slots to plug into the PCI slot, for example like this.



http://images10.newegg.com/NeweggImage/productimage/14-130-188-06.jpg



and some of them have 3 like this



http://images10.newegg.com/NeweggImage/productimage/14-133-007-04.jpg



Does it really matter? Since both cards are PCI, shouldn't they both be able to fit inside the slot?|||First card is for 5 volts signaling



Second card is capable of both 3.3V and 5V signaling. It is a universal type card.|||Well, they are probably PCI-E cards and PCI-E comes in different speeds and even different power ratings, so they put the slots in it to prevent you from accidentally putting it into a slot that will damage the card. You still need to be careful, however, because not all PCI-E slots follow the convention and you might be able to put a card into a slot that will damage it, but that's mostly the case with older motherboards (more than a couple years old mostly).