There's more to fit than effective top tube and stem lengths:

1) Though he 56cm has a 10mm longer eff. TT, the seat tube angle is 0.5 degrees slacker. If you put the saddle in the exact same position relative to the bottom bracket for both frames, the saddle on the 56cm will acually boe more forward relative to the TT, making the TT effectively shorter. In other words the 56's TT will feel a little less than 10mm longer than the 54's.

2) Stem length itself is only relevant if the stem is parallel to the ground. If there's any angle, then the effective length of the stem is actually shorter.

3) The difference in height between the saddle and handlebars makes a big difference. Since the saddle position should be fixed relative to the BB, the only way to affect the relative height of the handlebars is the head tube length, spacers below the stem, and the rise of the stem (affected by the stem angle and extension).