speaker impedance

i have a pioneer vs500dvd mini hifi set. the main front speaker system consists of superwooferX2 and frontspeakerX2 (as shown in the pic).

the specification of these are as stated in the manual.

by physically looking at the label behind the superwoofer, the impedance is 8 ohm, while the impedance of the frontspeaker is 12 ohm. FYI, each of the speaker has its own set of cable. thus each has its jack on the main head amp unit

here comes the problem...
the manual states:
Nominal impedance (total) is 6 ohm.

how is the 6 ohm being calculated?

i'm worried about the impedance as i'm planning to salvage and hook these speakers onto a receiver that typically accept 6-8 ohm speakers.

please help.