Model loading and saving seems to change the model file

When I load a model:

model = transformers.AutoModelForSequenceClassification.from_pretrained("german-nlp-group/electra-base-german-uncased")

And then save it 2 times:

model.save_pretrained("saved_model-01")
model.save_pretrained("saved_model-02")

Both pytorch_model.bin files are equal (when I do sha256sum). When I load the model again both sha256sums are equal again but are changend from the first iteration.

Why is that the case? Is the saving should be deterministic or is there a date in the searialized data?