As far as I know, they do that without any bias... a lot of stuff they heaped praises on get dropped from the list b/c out-of-date-ness

From my understanding the "class" is relative. Meaning that a "class A" amplifier from 5 or 10 years ago might only be "class B" today because of advancements in the state of the art. Because of that they remove something if it's been on the list for 3 years unless a follow up review has been done or a reviewer for the magazine currently owns it because they don't know how it would compare to gear currently in that "class." Which does make perfect sense if you think about it.

Sadly because they don't have enough time or space to do lots of follow up reviews quite a bit of really good gear that should still be on the list falls off. The issue when a component is removed usually lists the reason for removal which could be anything from "not auditioned in too long a time" to "replaced by new model not yet auditioned" to "no longer available."