Macbeth interprets their speech to suit his pre-existing desires and therefore it could be argued that Macbeth twists their prophecies in order to further his own agenda of power
Paradoxical phrases are often used by Shakespare, when the Witches are speaking to demonstrate the ostensibly (seemingly true but not necessarily true) nature of their positive prophecies made to Macbeth
The serpent used untrustworthy language to tempt Eve, in the Garden of Eden, through twisting what God had said and claiming he didn't want them to become like him; the fruit wasn't really forbidden
The oxymoronic language used by Shakespeare reflects Macbeth's confusion in the audience, who would be similarly ambivalent towards the actual meaning of the prophecies, as they too cannot understand the oxymorons
Links to Banquo's later comment : "you should be women/And yet your beards forbid me from to interpret / That you are so."- The witches clearly, work beyond the natural laws of physics and biology