Caecilia is the default font used on the Kindle... duh!
BTW, If you explicitly install one of the default fonts using fonthack (i.e., have font folders "Caecilia", "Helvetica", "KindleBlackboxC", or "condensed"), expect inconsistent display.
Even if you don't activate them, they may interfere with your font selection. For instance, the mere presence of a "Helvetica" font folder, with Averia set as the SS font, will have part of the sans-serif stuff displayed using Helvetica, part using Averia. This happens because of how the overriding mechanism works. To avoid this, do as I said above: don't create these 4 font folders. Consider these 4 names as "taboo"
