When using sequel pro with a MySQL database, typing NULL into a string field will make that field null rather than 'null', which always seemed like an odd design choice to me
Its a good compromise i think. OTherwise to update a field to null it would need a button or right clicking and selected a null option. I would expect the amount of people who actually want to enter a 'null' string is minimal...
This reasoning is probably behind some the OP's cases...