Probably because the 17:56 from Reading overtakes it ... and reaches Temple Meads at 19:12, when the 17:36 is only just passing through Avoncliff ... it doesn't reach Temple Meads until 19:41, and I can't imagine customers who saw "Bristiol" on the departure board and jumped on it being very happy.
On the same basis, at Paddington you see trains to Radley and I suspect they go on to Oxford