Original: Historically the county town was Abingdon, but in 1867 the town of Reading - by then much larger - superseded Abingdon in this role. In 1974 local government reorganisation moved Abingdon and several other north-west Berkshire towns into Oxfordshire.
Simplified: In the past, a town named Abingdon was the county town, but this town is now not in Berkshire.