Why seconds instead of milliseconds?

#1 I will go with, this is something exposed to script, customizing weapons would be something modders would inheriantly do. Possible it is more friendly for more people to understand using seconds rather than milliseconds.

but only GG really knows

#2 I would go with something more like: programmers aren't the paragons of consistency they'd like to be.

anthony's explanation is way more generous.

actually, when i'm creating a value which will be exposed to users/modders, i do try to give it convenient units.
for example seconds over milliseconds, degrees over radians, antiwiddershins over clockwise, etc.

but most important of all is to document whatever units you are using, of course.

#3 That makes sense. Modding or just the fact that it wasn't consistant.