🚀 go-pugleaf

It is actually a little more complicated than that. If there is only one marker, the link includes all contiguous text with the same character format as the marker, in both directions, as long as it is in the same paragraph. If there is more than one marker, the link for the first ends at the second marker, the link for the second starts at that marker and includes all following text with the same format, up to the end of the paragraph or the next marker, and so on. So if there are no format changes, the link for an index entry with only one marker will encompass the entire paragraph. 

The rest of what Thomas said is right--if you change the character formatting of other parts of your index entries relative to the page numbers, a side effect will be a reduction in the "hot" area of the link. This is true in TOCs as well. I format my TOCs so all the page numbers are the same size, which means the text of some entries is not automatically linked. I then create new links for those entires by running an AppleScript that duplicates the hypertext markers into the differently-formatted areas of those entries.