In views that don't have data import enabled, Custom Dimensions values may be viewed for dates before the Custom Dimension was created.

When you define a custom dimension or metric, you specify its name and other configuration values, and Analytics assigns an index number that you use later to reference the dimension or metric. Note that Custom dimensions values can be viewed for dates before the dimension was created. Tip: Unlike other types of data, custom dimensions and metrics are sent to Analytics as parameters attached to other hits, like pageviews, events, or ecommerce transactions. As such, custom dimension or metric values need to be set before a tracking call is made in order for that value to be sent to Analytics.