Blame Wizards of the Coast. A lot of these didn't exist until WotC wanted to shoehorn D&D into Heroscape. Don't get me wrong, I love the D&D figures, but the classes they added was redundant. Many of these didn't exist until then. But I'll explain some of them as best I can.
As a quick summary, it comes down to thematic reasoning or prevention of bonding so units don't become too strong. Almost all of them have a different meaning of what they are. While some could be omitted, most just wouldn't make sense what they are class wise. You aren't going to call some random knight is a group of knights a champion. Because they aren't. For historic units, you have to have the correct name or there is a very good chance you will offend a culture. And many in which could be omitted is because of WotC wanting to make money of Heroscape while making a cashgrab of an old franchise with as little work as possible.
First off with Spellcasters, Warwitch was like a contest figure. It was actually made by a young girl. a long time ago. This is why it is special. Archmage was likely made so that Sonlen, who was already powerful at the time, could not bond with the Elf Wizards. At least this is my guess. Acolyte is different from a wizard in definition. They are not the same class. Battle Mage and Arachnomancers were D&D. But an Arachnomancer is specifically is a Spider Mancer basically. And a Battle Mage is a magic user who can use close combat. So this is different from a normal Archmage or Wizard.
Ranger - Rangers are no specifically archers. Rangers can use close combat and far away attacks. But again, this was because D&D. Ashigaru are not rangers. They are a type of soldiers specifically used in Japan.
Entitled - Admiral's are over captains. While a captain is a ship leader. An Admiral is a fleet leader and is over numerous ships and not just one. Daimyo is Einar specific because of theme reasons and the true title of what they were called in history. Darklord is for the werewolves mostly and is their thematic reasoning. Duchess, Lady and Lord are all Vampiric themed. Lord and Lady are the correct titles. Sonya and Cyprien are paired together. Duchess could have probably been omitted. King is a dragon only thing because that is what they are and what they called themselves in the lore. Leader is a D&D class and could have been left out. Warlord is very different from the others.
Champions are Knight leaders. Death Knights are dead knights. D&D term and could have probably be omitted but were probably called death knight to prevent bonding. Deathwalker is a very very different thing and has nothing to do with the others. And should have been under Artificial.
Artificial: Construct is a all inclusive term for machines and creations not mechanical. Deathreaver and Deathstalkers are very different figures. Thematical it makes sense. Deathreavers and Deathstalkers could have been constructs, but them having 'death' in their name is thematically accurate for Alpha Prime lore which is where they are from.
And while you didn't list it, I know another one and that is Rogues, Assassins, Ninjas and Pirates. Again this is thematic reasoning. Rogues are more like bandits and thieves on land. While Pirates are bandits on the sea. But you don't really call a bandit a pirate unless they are at sea. And Ninjas are a Japanese military class. As for Mika who is called an assassin.... Well she is a bit of an odd ball. She probably could have been called a rogue instead. But she isn't really a rogue or ninja. So they made her an assassin. But I do believe she could have passed as a rogue. Tanukis being a ninja is also odd as they are rogues. But they had training from a ninja jonin, so that is probably why they are called ninjas instead. Though honestly their bonding should be with Ninjas and Rogues.
