s.bernbaum
Mostly lurking
The +90 from building a cottage on a forested grassland is the result of removing the forest. This is referred to as "chopping" by many here at civfanatics. It is a one-time bonus. Usually the number is not quite so high, so I suspect you have -increasing modifiers such as buildings such as forges or factories, civics such as organized religion, or leader traits such as Creative, where leaders have double production speed of theatres (at least in vanilla).
Correct unless playing Marathon with Mathematics already researched. In that case all chops in the BFC are +90 hammers.