Of course, because economy awards are more available than Biz or F awards. It would be hard to come by 8 Saver Awards in Business (perhaps totally impossible on some flights because that many Saver seats aren't ever released for those flights, not even close in). I just booked for 2 in economy (no Biz or First class on that flight at all) on a very short AA flight using 9k Avios roundtrip and there are about 5 flights available per day and each flight has 7 out of about 20 seats available as Saver Award seats. So, I'm assuming that for flights with many more economy seats available (usually ~200 when going transatlantic) there will be more Saver awards available to begin with.

The trick here is getting to go in upper class seats as a group of 4 or more on the same flights - certainly doable as per many posters above (and also in my own experience) but needs work.