Because the 2nd outcome in their opinion on the draw is less likely. You have given West Brom a 3 goal start so Arsenal would need to win by 3 for it to be a draw on the handicap
In the 1st one your only giving West Brom a 1 goal start so betfair think that a 1 goal Arsenal win is more likely...
What your showing there is your either giving West Brom a 1 goal lead and then betting on the outcome or a 3 goal lead and then betting on the outcome. So you can bet on the draw. E.G. In the 1 goal handicap if you think Arsenal will win 1-0 then your in affect betting on the draw