Well, from year to year, players improve or get worse. Even with the exact same cast you can get signiaficantly better or worse results based on injuries, chemistry, off season progress/digress, etc.
Claiborne and Jones and Wilcox are playing better. Lee has been injury free and consistent. Collins and Thornton have added talent. McClain is injury free and playing at his highest level ever. Lawrence looks to be at similar form to 2nd half of last year, but we still need to see. We got rid of cancers in McClain and Hardy. Brown is a rookie playing like a 1st or 2nd round pick. And overall, the confidence is higher than last year as a group.
So it might not logically seem like we could be a good defense, and I am not saying we are a top 10 defense, but it is a possibility that is what we are or are becoming as the year moves forward.