You should definitely read the FreeType license if you want accurate information.
As far as I know, there are two cases:
1. Dual licensing. Because some may prefer GPL, and some others may prefer something else. Don't ask me why, I'm not a GPL expert. But according to FreeType license page, GPL v2 has some incompatibilities, even with more permissive licenses.
2. Sometimes, libraries are under a permissive license, but some (optional) parts of it, plugins or whatever, are under a different license, because they themselves depend on other libraries, or because the contributor insisted on using that license.
FreeType seems to be in case 1. so we can choose to use it under the FTL (BSD style, very permissive) and not worry about it.