Actually you are correct. I was surprised at first but just had a check on stats and his knocks away from home has come on flat decks or dead rubber games where even other players have stood up and piled on big centuries.Just for examples,
100 in SA- Smith batted at 6 with Clarke being a standout scorer with his famous 148 odd vs Steyn, Philander, Morkel
200 at Lords- Again Rogers got through the initial tough period of the game and then Smith went on to hit a double century.Rogers also scored 170 odd.
138 in NZ- Joe Burns scored 180 in that inning batting at the top order.
100s at Oval in 13 &15 tour - Again in a dead rubber game on a flat deck he smashed centuries there.
117 in SL- Marsh knock was arguably better and the pitch was easier for batting.Not to forget a dead rubber game.