Adding it to Gas Arrow lowers the damage ; Why?

Deadly Gas -> Supported Skills deal 25% less damage with hits ; 75% More magnitude of poison inflicted with skills

Gas Arrow -> "Does not hit, but poisons enemies as though hitting them "

Hit Damage -> "Any damage that is not damage over time is hit damage "

Poison -> " Any such modifiers that specifically apply to hit damage do not affect poison damage "

Gas Arrow WITHOUT Deadly poison -> DPS:4,388.59
ARROW
Main hand estimated poison damage per second : 1488.17 to 2506.07
POISON CLOUD
Damage per second 6013.99
Average damage per use 4176.38

Gas Arrow WITH Deadly poison -> DPS:3,292.39
ARROW
Main hand estimated poison damage per second : 1957.8 to 3296.92
POISON CLOUD
Damage per second 4509.55
Average damage per use 3131.63


The estimated damage is going up with the gem, but the rest of the numbers are lower
??????

I know im retarded, what am I missing ? Thanks.