Dear Andrew,
Lets say I came to your Form Page with Tracking Code 123 and navigated to Thank You. When you break down Thank You page with Tracking Code, you can see 123 because Tracking Code is a Conversion Variable and it persists till the end of the visit.
Now i come by directly typing the Form Page URL in browser and navigated to Thank You. When you break down Thank You page with Tracking Code, you can see only unspecified because there is no tracking code behind the scene.
To understand much on the None, Unspecified, other, kindly go through the link None, Unspecified, Unknown, and Other in reporting.
Also, avoid using Page Metrics like Page Views, Time Spent etc against Conversion Variables. Conversion Variables are best when it is used with Conversion Events.
Thank You
Arun