Synergies

It is quite simply because they are only estimates for the future, and any estimates may turn out to be wrong!

For example, we might expect that if we take over another company that in total we might need fewer staff and therefore save money (a synergistic benefit). However, whatever ‘research’ we may have done before buying the other company, it is only afterwards that we can be absolutely certain as to how many staff we need. It might turn out that we cannot lose as many staff as we were hoping and therefore the benefit may not be as great as we expected.