That is because some expressions designate rigidly by means of describing the designatum: e.g.    From Wordnik.com.  [Rigid Designators] Reference

Here are some considerations in favor of saying that an expression could be rigid de jure without directly referring to its designatum.    From Wordnik.com.  [Rigid Designators] Reference

On one such account, a rigid designator designates its designatum in every possible world containing the designatum and in other possible worlds the designator fails to designate.    From Wordnik.com.  [Rigid Designators] Reference

When we evaluate these sentences with respect to other worlds, we do not seem to admit that the designatum changes, as it would if names were non-rigid descriptions taking the proper scope in modal contexts.    From Wordnik.com.  [Rigid Designators] Reference

In other places, Kripke seems to have in mind another account of rigidity: one according to which a rigid designator designates its object in every possible world, whether or not the designatum exists in that world.    From Wordnik.com.  [Rigid Designators] Reference

The designatum must first be picked out in α by whether it meets the descriptive requirements (with or without the indexing) there in α: but the purpose of the indexing is to keep the term referring, with respect to other worlds, to that same item picked out originally in α by virtue of meeting the right requirements there.    From Wordnik.com.  [Rigid Designators] Reference

Nobilissimum designatum, participem quoque regni ani, successoremque creavit. '.    From Wordnik.com.  [The Letters of Cassiodorus Being A Condensed Translation Of The Variae Epistolae Of Magnus Aurelius Cassiodorus Senator] Reference

According to some philosophers (Salmon 1981, p. 33 note 35), it would be metaphysically possible to coin non-rigid directly referential expressions, by devising some means of assuring that they change their designatum from world to world; even this claim may be controversial (cf. Soames 2002, pp. 264 “ 5; King.    From Wordnik.com.  [Rigid Designators] Reference