How does navigation from Gallery view to Details view happen in a 'Start from data' Power Apps application?

Question

I have created a simple application in Power Apps, starting from data (TableA) stored in a SQL database, which provided me with the Gallery, Details and Edit screens which all work correctly, and when you select an item in the Gallery it takes you to the Details View as expected.

I then added another table (TableB) to the application and manually created 3 additional screens (Gallery, Details and Edit). The Gallery for TableB is set up exactly as the Gallery for TableA but when I select an item it doesn't navigate to the Details form and I had to add the code...

Navigate(TableBDetailsScreen, ScreenTransition.None, {displayItem:ThisItem})
```on the item's navigation button on TableB Gallery, and this code  
  

displayItem

  
This works fine!  
  
So my question is why does the Gallery for TableA which is created automatically by PowerApps, not need the same navigation code? How does it navigate to the Details screen? The code behind the arrow button's onSelect property is simply...  
  

Select(Parent)