What's a double room? In Australia, according to this, a room with _1_ bed in which _2_ adults are the rated capacity.
Of course, for much of the US (and as is apparent in the comments), a double room is a room with _2_ beds, with _4_ people the rated capacity.
OP doesn't state whether the guests were Australian or other.
If you search on the definition of double hotel room, you'll see that the confusion extends to single rooms as well, which can be a room with _1_ bed in which _1_ adult is the rated capacity or _1_ bed in which _2_ adults is the rated capacity.