Coding Stories VII - Add, Subtract the same value

Looking over an open source library I found the following code:

interval void SomeMethodName(FooIncoming that)
if (that.numAnswers > 0)  
{
  SupportClass.IListSupport.AddAll(this.answers, this.numAnswers, (IList) ((ArrayList) that.answers).GetRange(0, that.numAnswers - 0));
  this.numAnswers += that.numAnswers;
}
if (that.numAuthorities > 0)
{
  SupportClass.IListSupport.AddAll(this.answers, this.numAnswers + this.numAuthorities, (IList) ((ArrayList) that.answers).GetRange(that.numAnswers, that.numAnswers + that.numAuthorities - that.numAnswers));
  this.numAuthorities += that.numAuthorities;
}
if (that.numAdditionals > 0)
{
  SupportClass.ICollectionSupport.AddAll(this.answers, (IList) ((ArrayList) that.answers).GetRange(that.numAnswers + that.numAuthorities, that.numAnswers + that.numAuthorities + that.numAdditionals - (that.numAnswers + that.numAuthorities)));
  this.numAdditionals += that.numAdditionals;
}
...
}

What I loved in this code was:

that.numAnswers – 0
that.numAnswers + that.numAuthorities - that.numAnswers
that.numAnswers + that.numAuthorities + that.numAdditionals - (that.numAnswers + that.numAuthorities)

I don’t understand why a developer would make something like this. Any number subtracted with 0 will have the same value like before. Also if you add a number and after that you subtract that same value you will get the original value.
The code would be equivalent with:

that.numAnswers
that.numAuthorities
that.numAdditionals

The second thing that I noticed is the ‘that’ argument. I’m not a fan of this kind of names. Maybe a name like ‘incoming’ or something similar would be better.