It is believed to be because they had not physically entered the country when arrested.
We can reveal that one of those on the vessel and charged was a man with a murder conviction.
He had been released from a maximum security prison in Albania in January after completing a 15-year sentence for shooting a woman at her home in 1997 following an argument.
Her eight-year-old daughter witnessed the shooting.
It is understood the man, who the Sunday Express is not naming, is now in an immigration removal centre. It is not known if he has applied for asylum. The Home Office declined to confirm whether it was aware of his conviction.