As I said is how it happened, one side bats 50 and the other 20. It is a poor scenario as they are effectively two different formats and the shorter the innings, the more chance of an upset. Even 30 overs a side would have at least given a better game, as thrilling as the chase might have been.
The other main factor is the side batting first is setting itself up mentally for 50 overs a side, they don't know until they've batted that the opposition will bat only 20 and so can't adjust. Maybe England would have taken the odd risk or two more had they known the target would be that small, although maybe they should state the innings have to be within say 15 overs length of each other to constitute a match (fair contest)