Sandbox licenses of a higher type may be used to create lower type sandboxes

Note:

When creating a sandbox of a lower type, a license of a higher type will be used if no licenses of the lower type are available. Given this, a Full sandbox license may be used to create a Partial Data Copy, Developer Pro, or Developer sandbox if no license of that type is already available, or a Developer Pro license can be used create a Developer sandbox. This may lead to times where you will see one in use and not see that type listed. You should total your counts of each sandbox type, if one of the lower types of sandbox has more than the limit you see at the top of the page then the license for it is being provided by one of the higher sandbox types that will be listed as in use. 

For example:

An Enterprise organization has a limit of 10 Developer Sandboxes and 1 Full sandbox after purchasing a Full sandbox. They can create up to:

• 11 Developer sandboxes
• 10 Developer sandboxes and 1 Developer Pro sandbox
• 10 Developer sandboxes and 1 Partial Data Copy sandbox
• 10 Developer sandboxes and 1 Full sandbox

In the case of options a to c they would need to delete or refresh the extra sandbox in order to be able to create a new Full sandbox after the appropriate interval has passed for a sandbox of that type.