This issue clearly revolves around a subterfuge. The same water is
not being frozen under the same conditions, but the changes are deliberately
not mentioned in the wording of the question, leaving it to the reader of
the question to assume that identical conditions prevail. Not only may the
mass change, but so may dissolved gasses, suspended solids, etc. There is
also the question of what we mean by frozen. Do we mean ice on top, frozen
solid, ... There is almost no end of variables to manipulate to alter the
result.
A much more interesting issue is what happens to water between 4 C and ice.
Evidence now suggest a liquid/liquid phase change. See Nature 392, 164, 1998
for an article by Mishima & Stanley.