I think it's obvious what's happening. My theory on the agents/ultra agents:
The ultra agents is set in the distant future, long after the first agents and galaxy squad. It has already been proven that Ultra Agents is in the same universe as Galaxy Squad (It's just a few decades in the future, considering how Solomon Blaze has aged), so maybe it's in the same universe as the original Agents.
Maybe the ultra agents are part of the same military division as the agents, and the truck has just been upgraded.