"Instances" metric is not available for the Mobile Device Type dimension.
If you are using Workspace then you might be referring to the 'Occurrences' metric which would be more than Visits since it is Page Views with persistence taken into account.
Essentially, if you fetch "Instances' metric for Mobile Device Type through API, then it'd be same as 'Visits' as Kaushalendra mentioned earlier i.e. whenever a Visitor uses a Mobile Device Type to start a Visit, it'd count towards "1 Instance" and "1 Visit".
If you are trying to get the number of times your Visitors use Mobile Device Type to access your site then you can use either 'Instances' or 'Visits'. Both would get you the same insight.
This behavior is because of the way Mobile Device Type reports are populated. It is populated through a Third Party Device Atlas and is done on a Visit basis and hence Instances and Visits metrics provide the same numbers.
your calls look ok, you normally should see different data. what segment are you using? did you try the calls without the segment? maybe it's a hit-segment where only 1 hit per visit is returned (and therefore the data should be the same)....