Strategy Tester Period giving different P&L -- Why

Hi all, 

I have a sample EA with the following OnTick() logic getting a Custom (or any build in indicator like iMA(..) ) with PERIOD_D1. 

However, running Strategy Tester with Period set to "Daily" vs "M1" or other periods gives DIFFERENT Trading Backtest results. (I somehow felt that we should stick to "Daily" in the Strategy BackTester")

Can anyone explain why this is so?

int OnInit()

  {

//---

   controlPanel_ComboIndicator = iCustom(_Symbol,PERIOD_D1,"TALibrary\\iComboIndicator", 0, PRICE_CLOSE); // Trading 15-minutes windows

      }