The difference is that it is actually valid according to the HTML spec. Browsers will render lots of invalid HTML which is shorter than that, but if you put such HTML in a validator such as [1], it will have errors.

Putting <!DOCTYPE html><title>.</title> in that validator returns no errors (it does have a warning though).

Although in response to OP's question, no, I don't think it is related.

[1] https://validator.w3.org/nu/#textarea