[OS X TeX] ligatures

[Dr. Clea F. Rees]

If I typeset the following:
---ligature test file---
% !TEX TS-program = pdflatex
% !TEX encoding = UTF-8 Unicode
\listfiles
\documentclass[12pt]{article}
\title{Brief Article}
\author{The Author}

\begin{document}

f{}f ff shelf{}ful f{}f ff\\

\end{document}
---end test file---
then I get (what is to me) unexpected output.

Expected:
no-lig-ff lig-ff shel-no-lig-ff-ul no-lig-ff lig-ff

Actual:
no-lig-ff lig-ff shel-lig-ff-ul no-lig-ff lig-ff

Why does TeX ignore the instruction not to use the ff ligature in
'shelful' when a line break occurs afterwards? And why does it
nonetheless accept the instruction if the f-no-lig-f is typeset in
isolation?

Thanks,
cfr