They have to guarantee that the nutritional info is correct through the expiration date, but some vitamins break down over time quicker than others (putting in more vitamins than listed can help guarantee that there will still be enough after a year or two - another option is to not list a vitamin; then it doesn't matter if there's very little when you open a container years later.)
Companies can also list a "best by" date, but that's more about flavor.
Like @horsfield says, neither date means it will go bad by that date... It may last much longer, or may go bad sooner (unlikely.)
Any oil or fats can go rancid, some tend to go quicker than others. It also happens quicker when there's oxygen... There are three ways oil breaks down to become rancid: water, oxygen, or microbes... And microbes do best when there's water and oxygen. A powder necessarily has air mixed in, unless they vacuum-pack or displace the air with nitrogen.
So, some of it is how it's packed... Some is how sterile it is, going in... Some is the storage conditions... And there's kinda the luck of the draw.