Are you addressing this to me? I owned a Look size XL size 57cm frame which again is sized center of BB to center of horizontal top tube with 57.5cm top tube.
For the record. You are wrong. There is no size 56 in a Look as you say in your post above that you own. As I correctly stated and as reflected in the chart I attached earlier, Looks are sized in odd and not even numbers. You can't apparently read your own Look frame..Look frames have size decal at the bottom of the seat tube. Looks are NOT sized by top tube length but rather by effective seat tube length.

For point of clarification, the OP is right. He has a top tube length of 56. But, as he figured out, his Look is a size 55 which is a Large. Again, it is sized center to center which is the same size as a 57cm with more traditional center of BB to top of seat tube. Look opts for center to center sizing convention however.