I can think of 2 reasons why,
1. when you click "Start Reading" it starts you right from the bottom of the list so if the officials are at the bottom you read right from them and not the unofficial chapters.
2. the fan-tl is almost always far ahead of the officials so they tend to stick closer to the top as to avoid mixing with the official chapters.
also most uploaders (not all but most) label their chapters with "official" on them, so you're still able to tell right from the "Latest:" section under where it says status, author, genres & alt name, if an official chapters been uploaded since you'll still be able to see it at the top of the page even if it's not at the top of the chapter list. for me personally I always scroll the full list regardless of what kind of update the stories gotten but that's just me.
why are the official chapters usually at the far end of the list of chapters? is it coz they are later than the others? some readers didn't even know they exist until someone tells them