They are. And I see your point. Normally I would do the same. Not quite sure how on xGbuildup though (I dont have the spesific data).
But the fact that they include that game should probably favour Maddison. Remember, xGBuildup exclude direct contribution, meaning that Chilwells two assists and one goal in that match is excluded from the data. In other words, there are 6 goals left for him to contribute to in xGBuildup-model. But there was at least one penalty and one free kick. Four left, then. Maddison, on the other side, was only directly involved in one goal that game (I think), his own free kick. Meaning he potential could have been involved in 7 goals in the xGBuildup-model.
If you look at the figure last year, it was close between the two of them. 0,22 for Maddison and 0,20 for Chilwell. But Chilwell looks much more essential this season.