Converts to Catholicism are obliged to subscribe absolutely to what the Catholic Church teaches. Denial of even one doctrine or dogma implies that the convert does not accept what the Catholic Church teaches only what he/she wants to believe. This could extend to one teaching or to many.
It shows that the convert only becomes a Catholic on their terms, Tony Blair is a good example of this. Compare him to Anne Widdecombe who accepted the Faith in it's entirety.
I still await an explanation as to why someone who denies most of what the Catholic Church teaches imagines that they remain Catholic.
There are those, of course, who mistakenly believe that they can change the Church from within, they hang on. Others I liken to lapsed Jews who only come together at Passover and other festivals: religion never darkens their doorsteps in between.
Calling oneself a  Catholic for a lot of people is nowadays purely a cultural 'thing'.