Tototy that's an impossible question to answer with an absolute...

Tototy that's an impossible question to answer with an absolute response, all I can offer is possibilities.

Psychologically gaps occur most often as an overreaction or wrong reaction, fills simply correct that. In the case of BHP they may occur as a result of o/n commodities prices or trading on other exchanges (which is why so many major miners are full of gaps). Then of course it comes back to a fill because of the amount of attention they receive.

The above points are all valid reasons however they don't, in my opinion, explain why gaps are filled with such a high frequency and with such precision. That one I can't answer.