And selecting the initial 10 rows can be demonstrated to return the 3rd column using the initial default set in step 3. (It makes no difference if any rows are added between steps 3 and 4.)
This means that there *must* be two default values stored when a new column is added: one for the set of already-existing rows that don’t have the new column and one for any new rows. Initially these two default values will be the same, but the one for new rows can change (e.g. in steps 4 and 5 above) with breaking the old rows. This works because after the new column is added (step 3 above), it’s impossible to add any more rows that *don’t* have the new column.
And this is exactly how it works. Let’s investigate!
In typical Paul Randal fashion, this is both a look at internals and an interesting explanation.