Rosencrantz and Guildenstern were sent before Polonius was sent to try and get Hamlet to go to his mother. They seem to have come straight from that (failed) errand to Claudius. There is no discussion about it that we see – but they must have previously reported their failure to insure Hamlet’s cooperation. Perhaps their enthusiasm for the errand to England comes form an attempt to make up for the previous failure. As in, no, we couldn’t bring him to his mother’s closet but we CAN bring him to England.
Polonius, though he got no more of a guarantee of Hamlet’s compliance than Rosencrantz and Guildenstern did, reports compliance and assurance. He has a politician’s skill in this.