The preseason is one of the most misleading periods in football.
For the defense, they could've been trying new schemes or packages for the first time and guys were out of position. There can be endless reasons on why some units succeed and others do not in the preseason. I am sure some of the veterans don't want to get injured in meaningless games, so a missed tackle here or there is not that big of a deal as long as they are ready to go in Week 1.
There is very little correlation between the preseason and regular season records. I once read an article that showed that 2 preseason wins, on average, produced the best regular season records. NE is 0-3 and the Colts are 1-2. Cleveland is also winless, so take the records for what they are worth.