That's true, I used to quote .004" as the standard for seating primers, but I've found that some primers, and primer pockets, require deeper seating to be reliable, hence the variable. On the other hand, some primer/primer pocket combinations won't allow for the .006" seating depth without crushing the primer. At any rate, seat the primers to the bottom of the primer pocket and set the anvil correctly, and there won't be any primer issues, if the firearm is in proper working order.
Hope this helps.
Fred