It happens because there are different affixes. For example: Dead Man's Legacy indeed gives you +170–200 Dexterity and +170–200 Vitality, but it also gives you +1 Random Magic Properties. And that random property could have been Valiant for example, which gives the item +45-150 Dexterity and +45-150 Vitality. So a perfect roll of these stats would give you a Dead Man's Legacy with 350 Dexterity and 350 Vitality.

You can find the list of item affixes here: http://www.diablofans.com/topic/41045-spoiler-diablo-iii-item-affixes/

P.S. The very similar question has been asked and answered previously. Here is the link to the answer: http://gaming.stackexchange.com/a/73345/25490

It happens because there are different affixes. For example: Dead Man's Legacy indeed gives you +170–200 Dexterity and +170–200 Vitality, but it also gives you +1 Random Magic Properties. And that random property could have been Valiant for example, which gives the item +45-150 Dexterity and +45-150 Vitality. So a perfect roll of these stats would give you a Dead Man's Legacy with 350 Dexterity and 350 Vitality.

You can find the list of item affixes here: http://www.diablofans.com/topic/41045-spoiler-diablo-iii-item-affixes/

P.S. The very similar question has been asked and answered previously. Here is the link to the answer: https://gaming.stackexchange.com/a/73345/25490