Just from the numbers: the Instant Poison and Envenom changes are both +20% to those abilities, but those abilities contribute relatively little (around 12%) to overall assassination damage (source). Whereas Rupture being nerfed by a sixth has a proportionately greater effect since it does around 44% of overall damage (according to the same source). Of course, your damage distribution might be different if you're using different talents and artifact traits on beta from the simc model.
Indeed, there may be interactions that haven't been considered.