Why some Journal Dates get Parenthesis (and some dont')

Someone asked why some periodicals (ie magazines or journals) put the year in parenthesis and some did not. Good question. I looked into that. Page 22 of the pdf notes:

DATES: Abbreviate the names of months except for May, June, or July:
Jan., Feb., Mar., Apr., Aug., Sept., Oct., Nov., Dec. Journals paginated
by volume or issue call only for the year (in parentheses). [emphasis mine]

It has to do with how periodicals are published. Some may not record exact month, but rather record volume or issue numbers. For those put the year in parenthesis.

Can anyone further clarify this issue? Please do in the comments.

Images in this post take from the free Norton online 2009 update to MLA documentation.