Off Peak Day Return from Stevenage to/from East Grinstead (fare setter = Thameslink/Great Northern) allows outward travel after 0859 on a Monday weekday. So, you're good to use the 0901 from Stevenage.
The Off Peak (
period) Return from Stevenage to/from East Grinstead (fare setter = Thameslink/Great Northern) allows outward travel at or after 0930 on a weekday morning.
Off Peak Day Return / Off Peak Return from Stevenage to/from Headcorn (fare setter = South Eastern) doesn't allow outward travel on any train due into London before 1000. The 0930 departure from Stevenage (for Brighton?) gets into St. Pancras just too early at 0959, so that's why you had to wait for a slightly later departure.
Conclusion: There's no one standard definition of "Off Peak" for journeys from Stevenage. Depends on the exact ticket type, where you're going and, perhaps more importantly, as
@Jan Mayen has hinted at, who the fare setter is.