I have reached the end of my search.
Discovered the difference of 280 RP
I forgot to count the Glimmergate.
Overview:
Glimmergate: 280
Troll pit: 450
Tablet of tabernum: 290
Forgotten...
I don't know where the 280 RP difference comes from,
but it certainly isn't from the Gnomish Mine.
I expanded there, and didn't get any extra RP.
You were right, ShibuyaCloth