transwarp - How did the duplicate Voyager make it as far as the real Voyager?

The episode Course: Oblivion takes place in stardate 52586.3:

"Captain's log, stardate 52586.3. We've had a lot to celebrate lately – Tom and B'Elanna's wedding, Ensign Harper's new baby, and the continued health of our enhanced warp drive, which has taken us within striking distance of home."

The episode Dark Frontier, where the Transwarp coil was acquired, takes place in 52619.2:

"Captain's Log, Stardate 52619.2. We got another 20,000 light years out of the transwarp coil before it gave out. I figure we're a good fifteen years closer to home."

So actually, the real Voyager travelled the extra 20,000 light years after the events in Course: Oblivion.

Interestingly though, both production and release order were first Dark Frontier and then Course: Oblivion.