Derren, we've been around the block many, many times on this. It's the same reason an Ogre might be an Elite to a low-level character, a standard monster at mid-levels, and a minion with 1 hp at high levels.
The ogre is not actually getting weaker. Neither are the gold dragons. Their combat statistics - which are not actual descriptions of them, just descriptions of how they interact with PCs during combat - change based on the situation. In this case, their combat interaction in some insane fight vs. Bahamut makes it sensible for them to have these specific game stats. Out of that combat, they have whatever stats might be appropriate - most likely hugely powerful gold dragons.
Stats in 4e don't tell you anything other than how the monster performs in combats where the PCs are involved. I know you don't like this. You don't have to; nobody is making you like it. But every time this comes up, you put up exactly the same objections.
-O
Maybe you should read my posts better.
Its not a in combat / out of combat discussion, but a in combat with Bahamut / in combat without Bahamut thing.
Why would gold dragons be stronger fighters when they fight alone then when they fight at the side of their creator?
Yes, I know the answer. See Shazman's post.