Value/Growth comparison is done w.r.t. to broad market index or specific style indices?

CFAI-MOCK-PM-2011-Q21/22

Why is fund GB1 and GB2 compared to STI (Value weighted index of 30 largest firms traded on SGX) and not to SGX direclty (SGX is the Singapore stock market index)

I though General market index is better than comparing funds to the value index.