Which Series Are Graded Consistently And Which Ones Are Not?

I find that it is not a particular series of a coin that have a wide grade variance, but the age of the coin holder within a series as the grading standards have changed over the years.

This topic is a perfect segue to the problem of gradeflation.

Nothing is graded consistently unless you grade it yourself. We've seen dogs from every TPG in every issue. No two graders are the same.

Just remember: There's a person on the other end of that deal - at the TPG, and he or she is looking at hundreds of coins daily. They don't have an hour or two, usually, to sit and stare at a coin. On top of that, if a grade consists of (originality, strike, luster, surfaces, color, eye appeal), each of those factors might be subjectively weighted more or less by any given grader. One grader who dings a coin by a percentage of a grade point because of perceived negative toning might have the same coin graded up an equal percentage of a grade point because the other grader happens to like the toning in question. Same thing with fast-and-loose terms such as "market acceptable", a market acceptable old light cleaning on a Bust Half Dollar might drop the grade anywhere from 0 to 2 full grade steps depending on the preferences of that particular grader and his or her TPG. They can't even agree on what makes a Franklin FBL or a Mercury dime FB, between the TPG's.

In addition to that, a grader might see 10-20 Morgan dollars daily, but how often do they see things like chain and wreath large cents, pattern pieces, early date Proofs, territorial/pioneer gold, etc?

My experience has been that valuable and/or particularly rare coins tend to get more attention from the graders, although this doesn't always translate into a more accurate grade. For some reason, the TPG's in general seem to have trouble with old copper (pre-Lincoln), especially when you get down into Colonials and Early American. They seem to be fairly consistent with Morgans and Walkers. I see auctions all the time for Large Cents and Half Cents that appear to be under- or over-graded to my inexpert eyes, and other "subjective" issues such as coins that look original but got details-graded and coins that should be details-graded but got full grades.

The only thing truly consistent is the inconsistency. This is my conclusion. Until robots start grading coins, grading was, is, and shall remain an affair with some degree of subjectivity.

That is also one of the reasons that CAC exists, but even CAC is vulnerable to grader bias, and CAC has a less than neutral interest in their own product...