Thank you for choosing Doctorspring. I will be glad to help you.
I do understand your concern. Genetically,in case both parents have 'A positive' blood group their child can have either an 'A positive' or 'O positive' blood group. It is not possible for the child to have a blood group that is 'B' type.
It is possible that the results of the blood group might have been technically wrong. This could happen at any of the three points - your, wife's, child's. Blood group measurement is a manual process done under microscope . So error and confusion can come up.
Even after rechecking the results are same you should consider counselling and Genetic testing (DNA analysis) as this is only the fool proof way.
Hope this helps. I will help you out with any further queries.