A routine blood group test asks two questions: are there A antigens on these red cells, and are there B antigens? Someone with the Bombay phenotype has neither, so the answer comes back the same as it would for the most common blood group there is.
What the test does not ask is whether the H antigen is present. That is the foundation the A and B structures are built on, and it is what the Bombay phenotype lacks entirely.
Where the difference shows
Forward grouping — testing the cells — cannot tell the two apart. Reverse grouping can. Plasma from a person with type O blood does not react against other O cells. Plasma from a person with the Bombay phenotype does, because it carries antibodies against the H antigen those cells still carry.
Testing with anti-H lectin settles it. On ordinary O cells the lectin finds H antigen and reacts. On Bombay cells there is nothing to find.
Why this is not an academic point
A person recorded as type O will, in an emergency, be given type O blood. For someone with the Bombay phenotype that transfusion can cause a severe reaction. The misclassification is not a filing error — it is the mechanism by which the harm happens.
It is also why a previous transfusion passing without incident is not reassurance. Antibody levels vary, and a reaction can appear later than the transfusion that caused it.