Generally, dictionaries only have one form of a word (possibly: lowercase, Uppercase, or ALL) ranging from the most general to the most specific. To work with this constraint, check-spelling performs at least a case fold after checking for a perfect hit in the dictionary (this doesn't actually cover the Uppercase form, but it hasn't generally been a big problem).

Beyond that, the core for check-spelling assumed that it was useful to group related words together (this included plurals, possessives, and verbs in the past tense). Doing so could reduce the size of an expect.txt file, but it does lead to confusing behaviors when a root word comes/goes leaving only a plural form, or similarly, when one case of a word comes/goes while another remains.

If you're finding this behavior too confusing, or just want all forms to be listed individually, you can add:

with:
  disable_checks: word-collating

to your configuration.

There are actually two places where check-spelling does folding:

• dictionary checks - for this, the disable feature retains case folding but drops folding of ' and /[sd]$/.
• expected output checks - for this, the disable feature case folding as well as folding of ' and /[sd]$/.

cspell compact-dictionaries workflow