Django: model.objects.create() changes the previous model.objects.filter()'s resultset

Django: model.objects.create() changes the previous
model.objects.filter()'s resultset

I have a simple model named Keyword:
class Keyword(models.Model):
name = models.CharField(max_length=50, blank=False, null=False)
I'm trying to do bulk_create, but the bulk data may contain already
existing keywords, therefore I do the operation in two steps. First fetch
the existing keywords, then bulk_create new keywords. Following is a small
example of what I do in the process:
In [12]: t = ["aaaa", "dene"]
In [16]: existing = Keyword.objects.filter(name__in=t).all()
In [17]: new = set(t).difference(list(existing.values_list("name",
flat=True)))
In [18]: new
Out[18]: {'aaaa'}
In [19]: existing
Out[19]: [<Keyword: dene>]
In [20]: created = Keyword.objects.bulk_create([Keyword(name=name) for
name in new])
In [21]: created
Out[21]: [<Keyword: aaaa>]
In [22]: existing
Out[22]: [<Keyword: aaaa>, <Keyword: dene>]
Both bulk_create and create updates the previous filter's resultset. This
is most probably a bug, but I may also be missing some point here. Is
there anything wrong in my implementation?
Django version is 1.5.3.