A group designing a shared technical standard can have a large majority behind a proposal and still have one person point out a flaw that would break it. Rough consensus gives that objection more weight than a simple tally would. The aim is a workable decision after serious concerns have been considered, not a perfectly harmonious room.
Addressing an objection does not necessarily mean satisfying the person who raised it. A concern may lead to a revision, or the group may explain why it is outweighed or outside the decision's scope. Repeated disagreement does not automatically create a permanent veto.
Judgment, with a record
Imagine several libraries agreeing on a common borrowing-data format. One library identifies an accessibility problem; another simply prefers its current naming conventions. Both deserve a hearing, but they need not have the same effect on the result. A chair or other recognized process holder must judge whether unresolved concerns prevent progress.
That discretion makes the reasoning important. Without a visible account of objections and responses, “rough consensus” can become a dignified name for following the loudest participants or the chair's favorite. Participation also takes effort: those unable to track a long discussion may have their interests underrepresented. The process gains legitimacy from how it handles disagreement, rather than from an impressive percentage announced at the end.