Why do HMRC pursue odd tax cases?

Years ago I recall discussing HMRC prosecution policy with a senior official. He mentioned an occasion when he had lost a case and went back to his legal advisers to find out what had gone wrong.

"I thought you'd told me you thought we had a very strong case", he said. "So why did we lose?"

"Aha" said the lawyer, "You asked me what I thought and I told you that I felt that we had a very strong case. That was all you wanted to know. 

Had you asked I'd have told you that I also thought the other side had a very strong case too."