Okay, just a few possible explanations & ideas.
The number after the name, e.g. Thunderbolt VII, tends to mark an entirely different craft. The Thunderbolt I would be the American WWII fighter (P40?), the Thunderbolt II is the current A-10. The first Hellcat would be the F6F.
Different variations of the model, retain the same name, so the WCA Hellcat would have the same base designation in the WC3/4. The version is likely to be quite different. These can be designated by letter (thanks LOAF) as with the current US fighters, or other ways (Mk I, II etc.).
Just because we don't get to fly ships until a certain date doesn't mean they don't exist earlier. Also, just because we don't get to fly a ship after a certain date doesn't mean that it has gone completely out of production. Any given airframe will progress through several different variants in its lifespan, each with improved engines, weapons etc. There will naturally be a limit to how far you can push the model, however it is generally cheaper and easier to upgrade an existing model then start from scratch (only minor changes need to be made to the production line). An airframe will not be scrapped entirely until that limit is reached. Even once an air frame is 'superceded', some manufacturing may progress for use by militia and in backwater regions.
As I mentioned in one of my earlier posts, often rival companies will produce similar products. There is nothing that says only one light fighter, med. fighter etc. can exist at the one time. The Arrow/Hornet, Scimitar/Hellcat could easily be competing airframes available at the same time. Which you get to fly will depend on any number of reasons, which include in no particular order :
1. Which variant is better at the time.
An airframe may fall out of favor for a while, only to be given a resurgence by a new engine, shield generator etc.
2. Location.
Companies (and their airframes) may be more prevelent in certain sectors. e.g. despite being allies, the Germans/Japanese and US/British used entirely different aircraft in WWII.
3. Suitability.
Some aircraft may be unsuitable for use in some situations e.g. small carriers, or, for that matter, any carrier at all. The Hellcat is one of the few fighters that can be used in atmosphere. Perhaps it was originally designed as a planet/station based fighter, with only later varients modified for carrier use.