Hi Tony,
Bold text is thicker. The characters take up more space, both horizontally and vertically. The leading (space between the lines) may remain constant, but the height added to the characters means the line + leading can take about a point more vertical space. Bold characters are wider, so fewer of them (and fewer of the words they make) will fit between the margins. Some paragraphs will take an extra line, pushing all the text below down a line.
You could try decreasing the type size a bit. With 12 point Baskerville, single line spacing, and Pages default 1.0 inch margins, changing all but the five line paragraphs at the beginning and end of the page and from regular to bold pushed three lines of type to the next page. Reducing the type size of the bold portion to 11 pt kept the bold emphasis, but brought those three lines back onto the first page.
This won't work that well in many (most?) situations, but it should bring you closer to the same number of words on a page as you had before bolding.
Regards,
Barry
MacBook Pro, Latest version of Pages 6.2