postgresql EXPLAIN gives seq scan rows = 12 for filter id < 10

Question

I ran explain command on this query:

EXPLAIN
Select * from project_project where id < 10 FOR UPDATE

And the plan output:

"LockRows  (cost=0.00..6.57 rows=12 width=18305)"
"  ->  Seq Scan on project_project  (cost=0.00..6.45 rows=12 width=18305)"
"        Filter: (id < 10)"

How did the planner estimate rows=12?