When using Email Insights or other email tracking in Marketo, the device metrics do not always show the proper device that was used to open the email. Marketo tracks opens of emails thru a single pixel on the email that when images are loaded it tracks it as an opened email. However your stats don't always show the proper information or client.
Due to the way the pixel is tracked it pulls Email Client version data much in the same way as a web browser obtains what browser visited.
The "User Agent" as it is called is a widely adopted standard that email clients use to ID themselves. These strings/ID's are determined by the developer of the Email client. Some email clients have the same string for different operating systems (Gmail for Example).
Due to the user agent string, some situations may not always give accurate stats on the device or provider. Inaccurate data is less common with most major email apps and programs. The accuracy of data on some mobile platforms can be harder to differentiate due to the limited user agent data provided by the Email Application program.