A family of Microsoft word processing software products for creating web, email, and print documents.
Line spacing [aka Leading] is a Paragraph attribute which is a function of font size. The terms used (such as Single, Double, etc.) are not specific value settings. They're what I call convenience terms provided in the User Interface. It isn't possible to specify multiple Line Spacing values within the same paragraph.
One condition that can give the impression of mixed line spacing is when differing font sizes have been used in that paragraph. The standard Line Spacing value for "single" is 120% of the font size (although that can vary somewhat based on the discretion of the font designer). Accordingly, 120% of 12 pt is tighter than 120% of 14 pt., but both are single. If the font size varies the spacing for that line is adjusted based on the largest font size used, but the Line Spacing still is single.
Certain formatting attributes can give that appearance, as well. One such attribute is the setting to Raise/Lower selected text in Format> Font - Position.
Another, but less common, cause is when an image is inserted as In Line with Text. That can cause the line height of that line to be increased to accommodate the height of the image.
All of these conditions, however, usually are apparent - especially if the disparity is as great as you say. I tend to agree with Stefan, though... If you display the non-printing characters [¶] you'll most likely find that what appears to be a single paragraph actually is 2 or more. If so, it's entirely possible that the line spacing for each is different. As Stefan further pointed out, if they are separate paragraphs the gaps actually are caused by Paragraph Spacing Before/Spacing After settings.
Also, by having the ¶s displayed you may well find that the gaps are caused by the creator having banged on the return/enter key in an effort to force space by using "empty" paragraphs.