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