"ft" is a common ligature. When characters are often used together, fonts are often designed with those characters grouped together as their own character so they can be given certain design flair such as having the "branches" of the "f" and "t" as one continuous horizontal line. Aside from "ft," "ll" (ell ell) also often gets this treatment and others I can't recall off the top of my head. Text rendering engines will then substitute the composite character with the ligature if the ligature exists in the font being used for display. So what might have been happening is that the text engine thought that an "ft" ligature existed for the font it was using to show your book and was trying to show it, but in fact it didn't exist. But who knows at the end of the day.

More information about ligatures can be found on the Internet.