It's pretty simple.
You aren't changing anything. You are just editing the conversion options to STOP telling calibre to make a pagebreak at every <h2>
(Structure Detection is the setting in the conversion dialog that controls detection of chapters based on xpath.)
As it is, when you convert, calibre says "hey, <h2> tags! I should add a pagebreak (even though the original doesn't have one) and split the file here!" which in this case, you don't want.