The signals mentioned in the tables do not always have a value for each variable. Sometimes there is a value for one variable, but not for the other at some point of time. The upper table considers just the sequences of values for each variable (not synchronizing them to the same points of time) while the table in the lower part uses box-values to indicate that at that point of time, there is not value. The boxes are just used to indicate the absence of a value and this way, they synchronize the data streams into synchronous reactions (the columns). Without the boxes, the streams are desynchronized. They still have the same values, but their temporal relationship is lost.
