Scottie is pretty on the money, I would just stress the point that what we term the impedance of a speaker is in fact an average value and the reality is that a speaker's impedance will vary greatly depending upon the frequencies that it is producing.  A speaker that averages 6 ohms across it's impedance range does not present much more of a load to a receiver than an "8 ohm" speaker.  It is really only when you get to a speaker labeled as 4 ohms that you need to concern yourself with having an amp that is stable enough to drive it. 

As for why we design some speakers to be 6 ohms and others to be 8, we don't design our speakers to be a particular impedance.  We design them for sound first and foremost, then once we have gotten the sound that we desire from the speaker we take an impedance reading.  As long as the speaker's average impedance is above 4 ohms, then we go to production.  So really it's a consequence of getting the sound we want that causes some of our speakers to have different impedance values.  Sometimes I wish we just called them all 6 or 8 ohm since it really doesn't make that much of a difference, but we just aren't that type of company.  

Just let me know if you have any other questions, it's really no bother, thanks!