When these two values are the same, then your orderby statement does not tell how they should be ordered.
In the SQL world, order is not an inherent property of a set of data.
Thus, you get no guarantees from your RDBMS that your data will come
back in a certain order — or even in a consistent order — unless you
query your data with an ORDER BY clause.
The order may be different depending on storage engine, and so on.
PS. There is a chance that rows will be returned in ASC ID order, but it’s only a chance.