How Item Reference in Maintenance Schedule Item picking up data?

Hi all,

I need some clarification/explanation regarding Item Reference in Maintenance Schedule-Schedules.

I tried to create few Maintenance Schedule records to see and understand the flow. My query is on Maintenance Schedule-Schedules grid table. Example as below :

When we click any row to edit, below field is exist.

When I checked from back end site, those item code from tabMaintenance Schedule Item is not tally with tabMaintenance Schedule Detail. Output as below:


How does this is processed actually ? From my understanding, the item reference is generated automatically, but why it is not picking up/saving the same info ? And is there any significant impact if I set the item reference field property as hidden ?