Apparently you had a table with N variables but only assigned values to a subset of those when adding rows.
Warning: The assignment added rows to the table, but did not assign values to all of the
table's existing variables. Those variables have been extended with rows containing default
> In tabular/subsasgnDot (line 457)
In tabular/subsasgn (line 67)
As shown, it added the y value as asked, but filled in a zero (default value) for x.
Always add for every variable (even if want the default value) to avoid the warning. You could disable the warning if doing this deliberately, but I really wouldn't recommend that over explicit assignment.