Having a 53-man roster that’s evenly divided in talent is really just not a thing that happens. Inevitably, you are going to have some position groups that are stronger and some that are weaker, whether it is in the starting lineup or the available depth that you have.
For the Pittsburgh Steelers this year, they found themselves short on quality linemen and edge rushers, and so they acquired one of each via trade. On the other hand, they also had a surplus of talent alone the defensive line and at wide receiver, and so kept more than they usually do and still had to cut players who have been on their roster before.