Key takeaway
If two people need to compare notes to agree on a project's status, the tool has already failed at its one job.
It is a familiar and uncomfortable moment: a sponsor asks two people on the same project the same simple question, "are we still on track for the launch date," and gets two different answers. Not because either person is being dishonest, but because each of them is looking at a different, slightly stale version of the truth, one from a tracker updated Tuesday, one from a conversation that happened Thursday, and neither of them realizes the other's information is out of sync.
This gets blamed on communication far more often than it gets blamed on tooling, and that is usually the wrong diagnosis. People communicate as well as their tools let them. If the actual state of the project only exists by reconstructing it from several sources, disagreement between two reasonable, well-intentioned people is not a failure of either person, it is the predictable outcome of the setup they are working in.
What "single source of truth" actually requires
The phrase gets used loosely, but it has a specific, checkable meaning: one place where a fact about the project is written down once, and everyone who needs that fact reads it from there rather than keeping their own copy. Not one tool that everyone is technically logged into, one place where updating the fact once actually updates it everywhere it is used. A due date changed in a spreadsheet that a deck then quotes from last week's export is not a single source of truth, it is two sources pretending to be one.
Why this matters more as a project grows
On a small project with two people, the gap between what everyone knows and what is written down barely matters, someone can just ask. As a project adds people, time zones, and stakeholders who are not in the room day to day, that gap becomes the actual state of the project for everyone who is not the PM. Getting the record right stops being tidiness and starts being the only way most of the team ever finds out what is actually going on.