The gospel offers an atonement that is sufficient in value for all sinners. The invitation of the gospel is offered to all, because the atonement of Christ is sufficient for all. In a word, the atonement is unlimited in the value and worth of sufficiency. That's what A.T. Robertson meant when he wrote that it is "sufficient for all."
The atonement is sufficient in its scope, but it is also efficient in its application. It is applied only to those who repent and believe on the Lord Jesus Christ (Acts 2:38-39; Acts 16:30-31). For this reason, A.T. Robertson could write that the atonement of Christ is both "sufficient for all, efficient for some."
The atonement of Christ is sufficient in value for all. The gospel is offered freely to all. However, the saving power of the atonement is applied only to those who believe on the Lord Jesus Christ (Acts 16:31). Therefore, Dr. Robertson concludes that it is "efficient for some."