Typeface appears uniform in digital but is different weights when printed

Relative newbie to InDesign here; thanks for your assistance. I've got a simple InDesign document that I've converted to PDF. When viewed in ID and in the PDF, the typeface looks perfectly uniform throughout. Yet when I print the document, some sections of the text are heavier than other sections:

(I turned it sideways to emphasize the weight; red dots indicate lines that are heavier.) I checked every variable I know of but so far the only thing changing from line to line is the kerning - everythng else appears to be exactly the same (color is uniformly black, 13pt font, all Constantina regular).  Any ideas why this might happen? Thank you!