She was his daughter, and based on Lukes feats and Yodas yammering, she is also insanely powerful. It is fully possible her raw force powers, instinctively honed into an "you ain't seeing shit" effect over her entire life, are strong enough to bluff him.

The real cause is that she wasent his daughter at this point in the movies, because luke and leia weren't related in the first one. George "i want han solo to be a lizard man" lucas leaned in and made it "weird" after the fact.