Gokai will be a bit delayed this week, so have some more Agito.
So what's the deal with Agito and Kuuga? Why do they look so similar? Why do they keep referring to terms from Kuuga?
I think Decade explained this best. The story of Agito takes place in a world which seems very similar to Kuuga's world, but is actually completely independent. While they use some terms that are very Kuugaish, you'll note that they never mention Kuuga or Grongi. In fact, if you pay attention to the dates, there is no way that Agito and Kuuga can coexist, because their timelines wouldn't match up.
The explanation that I hear for this is because they wanted to do more Kuuga, so Agito started off as a pseudo-sequel. But beyond the first few episodes, you'll note that they never really mention anything Kuuga related again.