Here’s why :
In a standard WYSIWYG text editor, your left and right indents/margins are thought like this :
One on the left, one on the right.
But in Scrivener, the page is infinite. So there is no right side to be relative to.
(The “page” has a top and a left side, that’s it.)
So the indents/margins are rather thought/set like this :
Both of them are relative to the left side.
An ereader doesn’t work like that. So compiling you need to tell Scrivener what to do now that there is a right side to actually be relative to.
Or in other words: you never actually had a right indent/margin prior to this point.