When Ash lost to Harrison, he had no clue what Regice was. When he first met it, Pikachu had fought many battles, so it's no surprise he was able to defeat Regice.
And there are no levels in the anime. Perhaps the Taillow that almost beat Pikachu was a really strong Taillow. Or maybe Pikachu was having a bad day and felt like slacking for whatever reason.
What's the point in arguing levels in the anime anyway? Anytime "aim for the horn" is a valid battle tactic, you gotta just toss the game's logic out the window and accept that if you aim for the horn, Electric attacks > Rhydon.