try to use the top tube length as a benchmark around which to 'compare'.
with a 'compact' frame (sloping toptube vs a horizontal TT) you can easily go the 56 with your inseam.

I would - triple check your inseam measurement...
it's not that uncommon to be that inseam short for someone 179 tall, but it is unusual enough to skew the fit calculator...
I like to use a 1/2 to 3/4 inch hardcover book - jam it right up there until it's tight up against the sitzbone (move everything else aside...). With you backed to a wall, the right angles of the book makes sure you're 'handgrenade' close to being level when the book is set against the wall. Make a mark at the book edge and measure from floor.