Alias of VALUES RTE in explain plan

While reviewing Richard's patch for grouping sets, I stumbled upon following explain output

explain (costs off)
select distinct on (a, b) a, b
from (values (1, 1), (2, 2)) as t (a, b) where a = b
group by grouping sets((a, b), (a))
order by a, b;
                           QUERY PLAN                          
----------------------------------------------------------------
 Unique
   ->  Sort
         Sort Key: "*VALUES*".column1, "*VALUES*".column2
         ->  HashAggregate
               Hash Key: "*VALUES*".column1, "*VALUES*".column2
               Hash Key: "*VALUES*".column1
               ->  Values Scan on "*VALUES*"
                     Filter: (column1 = column2)
(8 rows)

There is no VALUES.column1 and VALUES.column2 in the query. The alias t.a and t.b do not appear anywhere in the explain output. I think explain output should look like

explain (costs off)
select distinct on (a, b) a, b
from (values (1, 1), (2, 2)) as t (a, b) where a = b
group by grouping sets((a, b), (a))
order by a, b;
                           QUERY PLAN                          
----------------------------------------------------------------
 Unique
   ->  Sort
         Sort Key: t.a, t.b
         ->  HashAggregate
               Hash Key: t.a, t.b
               Hash Key: t.a
               ->  Values Scan on "*VALUES*" t
                     Filter: (a = b)
(8 rows)

I didn't get time to figure out the reason behind this, nor the history. But I thought I would report it nonetheless.