Why Zotero and EndNote exports never quite match the required style

When output looks wrong, the instinct is to blame the style file. Usually the style is doing exactly what it was written to do with the data it was handed, and the data is incomplete.

A CSL style is a set of instructions: put the journal abbreviation here, italicise the volume, use sentence case for the title. If the item in your library has no journal abbreviation stored, the instruction has nothing to place, and the style either falls back to the full title or leaves a gap. Neither is a bug in the style.

Reference managers format metadata. They do not verify it. Whatever was captured when you saved the item is what gets formatted, correctly, forever.

This is the single largest source of non-compliance in medical and chemistry submissions, because Vancouver needs NLM abbreviations and ACS needs CASSI abbreviations, and neither can be produced from a full journal title reliably.

Zotero’s own forums put it plainly: the abbreviation field is empty in most items, it is populated by the site translator from whatever metadata the publisher page exposed, and it is the most inconsistently provided and formatted field across publishers and sites. Common failure modes include the field being empty, or containing a copy of the full journal title.

Zotero can auto-abbreviate in Index Medicus/MEDLINE form when the field is blank, which helps for medicine and does not solve ACS, whose abbreviations carry periods that the medical convention omits.

• Case. APA wants sentence case for article titles; MLA and Chicago want title case. Managers store the case that was captured. Databases export in every combination, and a style cannot reliably recase a title it cannot parse - especially with proper nouns, acronyms and species names in it.
• Author name parsing. "van der Berg", hyphenated surnames, Chinese names given family-name-first, and single-name authors all get split wrongly on import, and the style then formats the wrong halves.
• Item type. A conference paper saved as a journal article will be formatted as a journal article, flawlessly and incorrectly. Preprints, theses, standards and datasets are the usual casualties.
• Style currency. The Zotero style repository is community-maintained. A style can lag an edition change - APA 6 to 7 removed publisher location and changed the et al. threshold - and an out-of-date style produces consistent, obsolete output.

For a small bibliography, fixing the metadata at source is the right move: correct the item in the manager and the style will render it properly everywhere it appears.

For a finished manuscript with a hundred or more references, the arithmetic changes. Auditing item metadata across a library is slower than correcting the output once, and the output is what gets submitted.

That is the case citation format correction is built for: it works on the document rather than the library, standardises what is actually in the reference list, and leaves the body text frozen. Only the citations are processed, so nothing you have already settled in the argument can shift while the reference list is being fixed.