Citation Issues

When is the event date in parentheses?

I have a question. I am looking at the citations to vital records in EE.

Some have the vital event date in parentheses. See, for example, pp. 311, 313, and 315. The dates (1890), (1854), and (1727) are enclosed in parentheses.

Others do not have the vital event date in parentheses. See pp. 436-7 where dates 1871 and 1861, and 1895 are not in parentheses.

When are parentheses used? What is the principle involved?