Simple really. Just look at the departure time. That aircraft and crew have already been flying all day. Any delays during the day are cumulative (and there are many oppurtunities for delays) therefore an evening departure will invariably be late. They only allow 20 minute turnrounds in Scotland - so no way time can be made up on the turnrounds. They allow 25 minutes in Luton but Luton is probably the worst place for turnrounds.