This question is about a book 1/season 1 event, regarding enlightening from book 5. The event of the book 1 will not be in spoiler tags as it happens fairly early, but the reveals from book 5 will.
In book 1, following an order from King Robert, Varys sends a poisoner to sell some poisoned wine to Daenerys.
Jorah Mormont, who by then has fallen in love with Daenerys (come on, this is so obvious that it's not a spoiler..), saves her by spotting the seller's suspicious behavior.
In book 5, it is revealed that
Varys is working in secret with Illyrio Mopatis for years in favor of the Targaryens.
I put it in spoiler tag but it can be deduced as early as book 1, during the "Arya chasing the cat into the castle's underground" chapter.
What is really confirmed in book 5 is that
Mormont was a spy for the council. He betrayed it when he felt in love with Daenerys.
Since Varys could not have possibly known that Jorah Mormont would save the day, why did he actually send the poisoner instead of just saying to King Robert "I sent him, but he failed" ?
Am I missing something ? Let me kknow if the question is unclear.