Dextrocardia in mirror twin

On June 24th of this year I gave birth to identical twin boys. (Mono/Di) They were 8 weeks premature and were born to Twin to twin syndrome with a large size difference. (1 lb 5 oz)  We learned that our donor twin also was born with dextrocardia. HOWEVER- He only has dextrocardia. The rest of his organs are in the proper places. We were told we had mirror twins, however I am confused. How are they mirrored identical twins if only his heart is on the opposite side? Shouldn't he have situs inversus as well if they were real mirror image? They are no doubt about it identical twins. We had zygosity testing done, because I stubbornly refused to accept that they could be identical if only one had this problem.  The specialist we took him too said that he'd only seen two other cases of dextrocardia before and never in twins so he couldn't answer our question. I'm very curious about this!

So I guess my question is, why does only one twin have dextrocardia? And if the answer is indeed "mirror twinning" why is it only dextrocardia and not dextrocardia with situs inversus? Thanks in advance!