Small companies are consistent without trying, because the same person makes most of what customers see. Consistency becomes an active problem at the point where that stops being true.
Volume of output is what breaks it
As a company grows, material is produced by more people, in more places, often under deadline and without reference to anything produced elsewhere.
Each individual piece is reasonable, and the accumulated set is inconsistent because nobody was comparing them.
The drift is gradual, which is why it is usually noticed only when someone assembles a year of output and looks at it together.
Guidelines fail where they require interpretation
Documents describing a brand in terms of values and personality give a writer or designer nothing to check their work against.
Two competent people reading the same description will produce different outputs, both defensible, which is exactly the outcome the document was meant to prevent.
What functions instead is specific: defined colours, fixed typefaces, decided spellings, and examples of correct and incorrect use side by side.
Templates carry more of the load than rules
People follow the path that is easiest at the moment they are working, and a ready-made template is easier than reading a guideline document.
Where the compliant option is also the fastest one, consistency happens without enforcement and without anyone needing to care about it.
This is why investment in shared assets generally produces better results than investment in longer rulebooks.
Local exceptions accumulate for good reasons
Regional teams, partners and specific channels all encounter situations the central rules did not anticipate, and they solve them sensibly.
Each exception is justified individually, and after enough of them the central version is no longer what most customers actually encounter.
Companies that provide a route for exceptions to be requested and incorporated keep control, while those that simply prohibit them lose visibility of what is being produced.
Consistency is not the same as never changing
Identities are updated as products and audiences change, and refusing to move eventually makes a company look dated rather than dependable.
The distinction is whether change is decided centrally and rolled out deliberately, or whether it arrives through accumulated drift that nobody chose.
Customers notice the second kind as a vague impression that the company is less organised than it was, which is difficult to attribute and expensive to repair.