This is specific to the import flow.
Because there is no clock associated with LUT outputs, the switching information is meaningful only in terms of signal_rate.
Report Power reports the switching information in signal_rate and no clock information or toggle rate. This is clean.
On the other hand, XPE deals with toggle rate, even for LUTs.
These frequencies appear to be random (by keeping the toggle rate to 100% and deriving the clock frequency), but the creation of the .xpe file is done in reverse, by starting with the total power, LUT count, and 100% toggle rate, and then calculating the matching clock frequency.
This can be confusing in the case of multiple clocks, as the clock frequency will likely not match any clock in the design.