[garbanzo]

the headings were different.

here is the one that showed up as serif:

{\rtf1\ansi\ansicpg1252\deff0\deflang1033{\fonttbl {\f0\froman\fprq2\fcharset0 Cambria;}{\f1\fswiss\fprq2\fcharset0 Calibri;}{\f2\froman\fprq2\fcharset0 Times New Roman;}{\f3\fnil\fcharset0 Calibri;}}

and the one that showed up as sans serif:

{\rtf1\ansi\ansicpg1252\deff0\deflang1033\deflangf e1033{\fonttbl{\f0\fnil\fcharset0 Calibri;}}

turns out the first one had chapter headings in Cambria while the body was in Calibri. so the first font used in the file was Cambria.

i changed the font of the first to Cambria, and the reader displayed it using the serif font. yay!