It’s all a bunch of crap that needs to be reported accurately. I remember the old days when early previews (before midnight) were not rolled into opening day numbers. WB tried to do it for Matrix Reloaded and the administrator of Box Office Mojo refused to go along with it. He posted the Wednesday preview number ($5 million) completely separate from the opening Thursday, even though WB wanted all of it rolled together. Similar deal for Pirates 3, which had 7 PM shows on Thursday evening and BOM reported the $13M preview number separate from Friday’s gross, even though Disney wanted it all lumped together.
If you go way back in time, you can find Thursday previews reported separately on movies like Batman 89 and Jurassic Park. When the midnight massacre happened with TDKR, the entire industry decided it was a good time to start including Thursday shows as part of the Friday number. It’s a bunch of BS.