Brand guidelines are written for the situations the author could picture. Inconsistency almost always appears somewhere else, in a format that did not exist when the document was signed off.
The document describes a finished state
Guidelines show the identity applied correctly to a set of examples. They are a specification of outcomes rather than a procedure for reaching them.
That works while the situations resemble the examples. A press advertisement and a brochure are close enough to what was drawn.
A vertical video overlay, a chat notification, an invoice template and a job listing are not, and each is produced by someone with no design training.
Most brand output is made by non-designers
The volume of branded material a company produces is dominated by things assembled in a slide tool or a form field.
Those producers have a task and a deadline, and the guideline is a large file they have never opened.
Behaviour follows the path of least resistance, which means copying the last thing that looked acceptable, errors included.
Each copy carries the previous copy's mistakes forward, so a single early deviation propagates through hundreds of later documents without anyone deciding to change anything.
Templates outperform rules
A rule requires the producer to know it exists, find it and apply it. A template requires them to do nothing at all.
Wherever a format is produced repeatedly, a locked template moves compliance from a matter of memory to a matter of default.
This is why organisations with modest guidelines and excellent templates look more consistent than ones with the reverse.
The edges move constantly
New formats arrive faster than guidelines are revised, since revision is a project and format changes are announced by platforms.
The gap is filled by improvisation, and improvised decisions become precedent because the next person copies them.
Maintaining consistency therefore requires someone with a standing responsibility to add cases, not a periodic rewrite of the whole document.
What the document should contain instead
The useful core is short. Which elements are fixed, which are flexible, and what the brand must never look like.
Stating the flexible parts explicitly matters more than stating the fixed ones, because uncertainty is what drives people to invent.
A page a producer can read in two minutes changes more output than a hundred-page specification that is technically complete.