  The quantum uncertainty principle famously predicts that there exist
measurements that are inherently incompatible, in the sense that their outcomes
cannot be predicted simultaneously. In contrast, no such uncertainty exists in
the classical domain, where all uncertainty results from ignorance about the
exact state of the physical system. Here, we critically examine the concept of
preparation uncertainty and ask whether similarly in the quantum regime, some
of the uncertainty that we observe can actually also be understood as a lack of
information (LOI), albeit a lack of quantum information. We answer this
question affirmatively by showing that for the well known measurements employed
in BB84 quantum key distribution, the amount of uncertainty can indeed be
related to the amount of available information about additional registers
determining the choice of the measurement. We proceed to show that also for
other measurements the amount of uncertainty is in part connected to a LOI.
Finally, we discuss the conceptual implications of our observation to the
security of cryptographic protocols that make use of BB84 states.
