Question about template_preprocess functions
Yes definitely it will go to parent function too because this is concept of overriding. But final impact would be of your override function.

Say example: 

// Parent Function: First go to this fucntion

function template_preprocess_field(&amp;$variables, $hook) {
   print "Parent Function";
   $variables['test_variable'] = "Parent data";
   ....
}


// Child Function: Override values using child function

function mytheme_preprocess_field(&amp;$variables, $hook) {
   print "Child Function";
   $variables['test_variable'] = "Child data"; // Override value here
   ....
}


Now this $test_variable contains "Child data" value not "Parent data" because it is overrided.

I hope it would help you to understand basic concept of overriding.