If you have them mixed and you motherboard only supports one typ then it will only report one stick, e.g. the one it supports.
Though I'm surprised that it even boots as some MBs can baulk at mixing the two types.
CPU Z is a more detailed examination of your system than simply if it supports it, so it may well be able to see both sticks, that doesn't change the fact that the MB and consequently the BIOS only sees one stick.