  We point out that the relative Heisenberg uncertainty relations vanish for
non-compact spaces in homogeneous loop quantum cosmology. As a consequence, for
sharply peaked states quantum fluctuations in the scale factor never become
important, even near the bounce point. This shows why quantum back-reaction
effects remain negligible and explains the surprising accuracy of the effective
equations in describing the dynamics of sharply peaked wave packets. This also
underlines the fact that minisuperspace models ---where it is global variables
that are quantized--- do not capture the local quantum fluctuations of the
geometry.
