It's more likely that BA are getting much better than they used to be at managing their business. The A320 is a heavier aircraft than the A319, and they will almost certainly have negotiated a deal at LBA where they are paying no fixed landing fee but a per-passenger charge. They will therefore lose less money sending an A319 to Dublin (where they pay weight-based landing charges and it's a longer sector) with 60 pax aboard and an A320 to Leeds with 35 pax aboard where the more expensive A320 operates the shorter sector and doesn't get charged more at LBA for landing fees?