Yes, subtle and the reason that I (almost) never use a constant in the ON. By adding one more row to the original query it becomes clearer:

create table a (a1 int primary key, a2 int)

create table b (b1 int primary key, a1 int)

insert a values (1, 10)

insert a values (2, 20)

insert b values (2, 2)

insert b values (20, 1) -- The new row

select *

from a

left join b

on a.a1 = b.a1

and a.a1 = 1

drop table a

drop table b

a1 a2 b1 a1

--- --- --- ---

1 10 20 1

2 20 NULL NULL

This makes it a little easier to see that the ON is controlling which b rows join a, but not filtering the result set. (So, this is positive reinforcement to not do this.)