The hope of Claudius and of Polonius was that this arranged interview between Hamlet and Ophelia would either clearly indicate Hamlet's love for Ophelia or that he was definitely mad. However, Claudius reflects following what he has witnessed that neither is the case. Note what he says based on what he observed of Hamlet's behaviour:

Love? His affections do not that way tend,

Nor what he spake, though it lacked form a little,

Was not like madness. There's something in his soul

O'er which his melancholy sits on brood,

And I do doubt the hatch and the disclosure

Will be some danger...

Claudius thus comes to the conclusion that Hamlet's strange behaviour is not the result of thwarted love for Ophelia or madness, and believes therefore that it is the result of some desire or plan to cause danger to himself and to his own position. It is clear that Claudius suspects Hamlet may have discovered the truth of what he did and that now he is acting to oppose him and to take back the crown that Claudius took through killing Hamlet's father. This is why he resolves to send him to England.