What was the reason for changing the content rating system?

Derpibooru had Safe, Suggestive, Questionable, Explicit  
Safe was normal safe stuff  
Suggestive was saucy stuff  
Questionable was topless stuff/borderline explicit stuff  
and Explicit was sexy stuff/nudity/fully explicit stuff
 
Now here on Furbooru we have  
Safe being safe  
Suggestive being topless  
Questionable being full on nudity/sexy stuff  
and Explicit being fully explicit stuff
 
What was the point?

The target audience is different from the sites.
 
Derpibooru is tied up to the children’s show MLP therefore it is understandable a good portion of the audience is still going to be underaged.
 
Furbooru however is a multi-fandom site with focus on the furry community and from that we can assume the main audience to be older therefore the ratings were adapted to this reality.
 
It is also worth noting that despite the two sites sharing part of their staff they are still treated independently from one another hence have their own policy.