I believe it realtes to how the oneworld airlines divvy up the fares.
The lions share goes to those carriers doing the intercontinental flights and (I believe) contractually there must be at least three of these between TC1/TC2/TC3.
In this case to have the ORD/DEL flight skipping TC2 would invalidate the contractual arrangements - hence the 'special' "Hey look down - there is Europe" rule.