2.4% is the gross gradient requirement for a twin engine with one engine out in the second segment. It's 2.7% for a three-engined aircraft and 3% for a four-engined aircraft. The second segment lasts from the end of gear retraction to the higher of 400 ft AAL or acceleration height. The 3.3% is the nominal gradient for SIDs. If it is higher than that due to airspace requirements or obstacle clearance, the required gradient will be on the map.
2.4% is a certification requirement, 3.3 is the gradient used for procedure design.