Phyrexian Mana // Mana Curve

I was building a deck and noticed that Phyrexian Metamorph and Birthing Pod appear on the mana curve breakdown as being convertet mana cost 5. I checked a few other cards and it seems that each card counts as being one converted mana cost more than it really is. It does not appear to be per phyrexian mana symbol though, as Dismember appears as converted mana cost 4.

Just thought I'd let you know :)