I have an after update trigger that is firing 3 times when I run my test class. It only fires twice when I update the record through the UI. I'm stumped!

When I run my test class, the very first time the trigger fires, trigger.New and trigger.Old values appear to be the same, which is not what I expect to see in the debug log. The second time it fires I can see that the new and old values are different, which is what I expect. I know that one of the additional trigger runs is due to a workflow field update on the Account, that's fine. 

I can control the recursion through a static variable in my class, as I would normally do, but the problem here is that the recursion control stops the trigger after the first run, and in the first run the old and new values are the same so my trigger logic doesn't work as I would expect.

I've stripped out most logic from the code so that now it's just a plain vanilla textbook setup. 
 

trigger AccountTrigger on Account (after update) {
     AccountHelper.runUpdate();
}

public class AccountHelper {

     public static void runUpdate(){

              system.debug(trigger.New[0].custom_field__c);
              system.debug(trigger.Old[0].custom_field__c);

     }
}

@isTest
public class AccountHelperTest{

     private static testmethod void testThis{

           Account a = new Account(Name='TestAccount');
           insert a;

           a.custom_field__c = 'Changed';
           update a;
      }
}