The explaination should have been:

Since col2 is used in the WHERE clause and happens to be a null value and since null cannot be compared to other values, the col1 value 4 is not brought in by the query.

I'd come across such a situation while analyzing one of the queries (which I hadn't noticed until later) and was puzzled why the query wasn't bringing in the data even though all the join conditions were met.

This was a nice thing to learn!

Thanks to all for the inputs! 🙂