Here is what they mean. In any semiconductor manufacturing process, there is a rate of manufacture defect involved. Even if it's a fairly low rate, if you spend all the time and effort required to make the monstrous wiz-bang, 8-processor Cell, every time there's a defective processor it ruins the entire Cell of 8 processors.
Say for the sake of illustration that 1 percent of spe's turn out defective. For an entire Cell, you run that risk 7 times. The chances of at least one defective SPE ruining the Cell would be P = 1 - (probability of no defects)
= 1 - (.99 ^ 7)
= .0668 or 6.68 percent. Ouch!
That's expensive, to have to throw away that many expensively produced Cells. But how about this said the designers. Just make the Cell run on 1 People PC cpu and 6 spe's. For the 6 spe's we'll be able to ignore the defective one if there is one, and if there isn't, we'll just pick 6 of them.
Now there's only a problem if at least 2 of the spe's are bad. Still supposing a 1 % failure rate for SPE's, the odds of at least 2 being bad in the same Cell are
1 - (P: none are bad) - P(exactly one is bad)
= 1 - .99^7 - 7 x (.01 x .99^6)
= 1 - .932 - .0659
= 1 - .997
= .003 or about .3 percent.
This means that the vast majority of the time, if there's one bad spe in Cell, there's only one, not two. I like to double check so here's the probability of exactly 2 bad spe's.
P(2 bad spe's) = (7 C:2) (.01^2 x .99^5)
= (7!/(2!5!) x .000095)
= 21 x .000095 = .0019 or .19 percent. Less than .3 percent, it checks out. The difference is the probability of 3 or more bad spe's.
Now keep in mind that figure of a 1 % failure rate for the spe was just an example. It shows how the chance of 2 or more bad spe's is drastically lower than the chance for 1 bad spe. Because they are putting 7 spe's in each one, using 6 instead of 7 is a small price to pay for having far fewer units turning out defective and having to be replaced.
As for your other question, the Cell actually has 8 SPE's in addition to the powerpc cpu. One of them is chained to the cpu and can only be used by the operating system, so it doesn't really act as an spe. The one that is locked out is one of the remaining 7. So the ps3's Cell has a total of 6 spe's available, even though there are 8 there.
Now my calculations were for locking out 1 of 7, not one of 8 which is really what's done. I don't feel like re-writing them but they would show the same point, which is the chance of 2 defective spe's is far less than the chance of only 1, making it a cost-effective move. If you know probability and statistics you can go through and rewrite them I guess, but it holds true for 8 spe's as well as 7.