I am not quite sure about your question. But i think your are wondering why there are different syntax styles in angular directives. First off all, please have a look at this post: Difference between double and single curly brace in angular JS? The answer explains the difference between {{}}, {} and no braces.
For your concrete examples, as in the documentation stated: ng-href requires a template (any string which can contain {{}} markup), while ng-if requires an expression – e.g. you may not write {{}}, because angular evaluates it.
If you have a look at the angular sources you will see, that ng-href uses attr.$observe while ng-if uses the $scope.$watch function. $observe is caled on every change of the attribute value. $watch is called, when the expression results to a new value.
But why these two different ways? I think one point is easier usage and code readability. Right now you can write:
<a ng-href="http://yourdomain.com/users/{{userId}}/post/{{postId}}">title</a>
As you can see, you only write an expression for dynamically inserted userId and postId values. If ng-href would also use the $watch function we had to write (don’t do this because it wil not work – it only demonstrates the difference):
<a ng-href="'http://yourdomain.com/users/'+userId+'/post'+postId">title</a>