Think they cherry picked that over the last 2 and a half seasons, which is mostly when we finished 11th and were terrible. I'm sure he scored a few against top 6 sides in his first season.
I also think it's a flawed statistic unless you're playing for one of the top 2 or 3 teams. Naturally we, being between 11th and now 7th in that time, aren't going to batter the top 6 like we might Sunderland or Bournemouth. And thus Rom is more likely to be isolated and getting fewer chances.
He's still managed some superb solo goals against top teams mind you. The one vs arsenal from the right wing and the Chelsea cup goal come to mind.