Companies with several business lines often carry a weak one far longer than a standalone version of it would have survived. Consolidated reporting is what makes that possible.
Blending conceals the distribution
A combined margin is an average, and an average is consistent with a wide range of underlying performance. One strong line and one loss-making line can report acceptably together.
Unless the segments are reported separately with fully allocated costs, nobody outside the company and few people inside it can see which is which.
The healthy unit is effectively lending the weak one credibility, and does so silently.
Shared costs are allocated by convention
Overheads such as facilities, executive time and technology are split across units by rules chosen for administrative convenience, typically headcount or revenue share.
Those rules rarely reflect actual consumption. A small, complex unit can absorb disproportionate management attention while being charged a small share of it.
Change the allocation basis and the ranking of units can reverse, which tells you the reported unit profitability was partly an accounting artefact.
Weak units consume the scarce resource, not the plentiful one
The important subsidy is usually attention rather than cash. Struggling businesses generate meetings, escalations and turnaround plans that occupy senior time.
That time is the binding constraint in most companies, and it is being spent on the part of the portfolio least likely to repay it.
Meanwhile the performing unit, which is not causing problems, receives proportionally less scrutiny and less investment discussion.
Exit is delayed by the story rather than the numbers
Closing or selling a unit requires someone to state publicly that an earlier decision was wrong, and the people best placed to say so are often the people who made it.
Turnaround plans postpone that admission at low apparent cost, and each plan buys another period of measurement before judgement.
Several such cycles can consume years, during which the unit's realisable sale value typically falls rather than rises.
Segment discipline is a reporting choice
The correction is structural in accounting rather than in strategy: report each unit with fully allocated costs and a capital charge, and review them against that standard on a fixed schedule.
Fully allocated numbers are always arguable, and the argument itself is useful because it forces explicit statements about what shared resources are for.
A portfolio where nobody can say which unit earns its capital is not diversified so much as unexamined, and the difference only becomes visible when conditions tighten.
Reviewing on a fixed schedule matters as much as the numbers, because a review triggered by poor results arrives after the point at which a unit could have been sold for a reasonable price.