Each of these bids are expressed as a monetary value (how much the completed goal is worth to the advertiser) to ensure campaigns are as cost-effective as possible. It’s important to remember that these amounts are not maximum bid amounts (i.e. your effective cost-per-X may exceed the value specified), but rather an indicator of your priorities and the value placed on each one.
Facebook optimizes delivery of your ads based on market competition, target audience, and the overall campaign objective. To maintain a greater level of control over their spend, advertisers explicitly define the values for one or more of the supported oCPM goals.