Year lines (called “issues” in the API) are an indivisible set of information containing the year, the mint letter, a variant comment, etc.
A coin in your collection is either assigned to a specific year line or “undetermined”. In order to keep the catalogue clear, we don't create year lines to record collections with only partial data, like known year but unknown variant. This may evolve in the future with a better way to record varieties, but no such thing is planned yet. So, if the variant is unknown, your coin is recorded as “undetermined” and associated to no year lines.
Using the API, you may still get the year in such cases where the coin was issued during a single year. See the fields ‘min_year’ and ‘max_year’ in the response of GET /types/{type_id}.
