It doesn't mean that you should expect 20 mins delay on every arrival.
Quite so, however, it also means you could get 20 mins delay on every flight.
harry
Yes, you literally will have 20 mins when you arrive. However, when you leave your departure airfield, how do you know that you will be able to commit or that you will have an EAT. If you don't, and I don't think that you can say that, then carrying the 20 mins extra is, IMHO, a reasonable justification.
Having said that, I use my experience and judgement to decide on the day. That's what I'm paid for.
Regards,
H-D