Django продолжает создавать новые миграции, даже если модель не изменилась

Question

Вот моя модель:

class Biovariable(models.Model):
    bioid = models.AutoField(primary_key=True, unique=True)
    name = models.CharField(max_length=200)
    description = models.CharField(max_length=2000)
    units = models.ForeignKey(unit, on_delete=models.CASCADE)
    def __str__(self):
        return self.name


class Biovariable_data(models.Model):
    biovar = models.ForeignKey(Biovariable, on_delete=models.CASCADE)
    value = models.CharField(max_length=2000)
    evdate = models.CharField(max_length=30)
    evtime = models.CharField(max_length=30)
    linkedcustomer = models.ForeignKey(customer, on_delete=models.CASCADE)
    linkeddoctor = models.ForeignKey(doctor, on_delete=models.CASCADE)
    linkedclinic = models.ForeignKey(Clinic, on_delete=models.CASCADE)
    class Meta:
        unique_together = [
            "biovar", "value", "evdate", "evtime", "linkedcustomer",
            "linkeddoctor", "linkedclinic"
        ]

Каждый раз, когда я запускаю makemigrations, django продолжает создавать новые миграции, и я не могу понять, почему.

joel@hp:~/myappointments$ ./manage.py makemigrations
Migrations for 'appointments':
appointments/migrations/0010_auto_20190310_2138.py
    - Add field dob to customer
    - Alter field biovar on biovariable_data
joel@hp:~/myappointments$ ./manage.py migrate
Operations to perform:
Apply all migrations: admin, appointments, auth, clinic, contenttypes, sessions
Running migrations:
Applying appointments.0010_auto_20190310_2138... OK
joel@hp:~/myappointments$ ./manage.py makemigrations
Migrations for 'appointments':
appointments/migrations/0011_auto_20190310_2139.py
    - Alter field biovar on biovariable_data
joel@hp:~/myappointments$ ./manage.py makemigrations
Migrations for 'appointments':
appointments/migrations/0012_auto_20190310_2139.py
    - Alter field biovar on biovariable_data
joel@hp:~/myappointments$ ./manage.py migrate
Operations to perform:
Apply all migrations: admin, appointments, auth, clinic, contenttypes, sessions
Running migrations:
Applying appointments.0011_auto_20190310_2139... OK
Applying appointments.0012_auto_20190310_2139... OK
joel@hp:~/myappointments$ ./manage.py makemigrations
Migrations for 'appointments':
appointments/migrations/0013_auto_20190310_2140.py
    - Alter field biovar on biovariable_data
joel@hp:~/myappointments$ 

Один из этих файлов миграции выглядит так:

# Generated by Django 2.1.3 on 2019-03-10 16:09
from django.db import migrations, models
import django.db.models.deletion
class Migration(migrations.Migration):
    dependencies = [
        ('appointments', '0010_auto_20190310_2138'),
    ]
    operations = [
        migrations.AlterField(
            model_name='biovariable_data',
            name='biovar',
            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='appointments.Biovariable'),
        ),
    ]