If you've been to TVTropes, you should be familiar with their WMG-page, so you know the drill. If not, explanation in the link. Short version, post random theories (no matter how wild) to try and fill in gaps in canon or fix inconsistencies within it.
I'll start off:
Porygon-Z in Oblivia's Past
So here we have it, one of the strangest face-palm moments in Pokemon, the completely modern, and possibly not even well-known (due to possibly being an in-canon unofficial modification) Pokemon exists in the temple missions of Oblivia's past.
Well, let's look at its dex-entry, its Platinum entry says that it was created to work in alternate dimensions, but did not work out as planned. It's method of evolution, the Dubios Disc is found in a Galactic hideout in Platinum, almost suggesting that this Pokemon was developed in an early attempt to find Dialga/Palkia , Pokemon which rule over time and space, in their respective dimensions.(and also suggesting that this may have been added to its backstory after specifically for Pt)
Now we find Porygon-Z not only in the past, but in a completely different location from where it was first created. Keeping the previous points in mind there are a few possibilities:
- Porygon-Z worked exactly as planned and entered Palkia and Dialga's dimension. This caused them to be separated from the normal space-time continuum of the Normal Dimension. They were attacked by Palkia and Dialga and those that managed to survive were actually scattered across many different dimensions and times, some landing in Oblivia's past, others in other alternate continuities such as the PMD series. The ones that were sent to Oblivia's past survived because the Steelhead saw the potential in using these creatures as weapons and took care of them. This particular explanation doesn't require Ranger to be in the same continuity, but at the same time, does not matter.
- Porygon-Z wasn't a failure and actually traveled between alternate dimensions, but not to the dimension that Team Galactic planned, but wound up in Oblivia's past
It's possible what enabled it to travel to alien dimensions were damaged during the attack against Dialga and Palkia in the first possibility, and in the second one, the Steelhead's control over the Pokemon would prevent them from running away.
(pssssh so much for Bulbapedia claiming Celebi to be the only possible explanation)