"File prevalence/reputation is low" with signed installer

[Alex999]

My software installer (INNO Setup) has been signed with trusted certificate, but Avast puts it into the sandbox and doesn't allow to run. Users of my software are confused with this and complain on virus threat. If "Autosandbox" mode is set to "Auto" (the default), Avast doesn't even allow to run this installer, and INNO setup file fails. Why?

Screenshot: http://i1148.photobucket.com/albums/o561/Dmitry_RK/avast.png

Using Avast Free 7