Roflnadal

The model-based may include positions that are unsuitable, irresposible, or impossible for a mgr to take.

For example:
As was said above the model may say to take a 50% position in a closely held stock, but if only 20% is available to the mkt there's no way a mgr could take this position (excluding leveraging it, but even that may be difficult for a security that's closely held)

Or the factor model may have a large short position included which would be irresponsible for a mgr to hold and possibly unsuitable under portfolio investment guidlines.