I'm not sure if this is how you were signalling
Oscar Swan mentions in his book that he finds it irritationg that some bike riders give the "car without turn lights" signal for a right turn. That is the left hand up.
I agree with him.
A right hand pointing right is instantly recognizable as a signal for a right hand turn.
The left hand up? The first impression it gives is the person is signalling a stop or waving. The fact that most most drivers once saw this signal in a drivers manual doesn't kick into the brain for a few seconds (if ever). Being "correct" doesn't make the signal effective.
In a similar misundertanding, I once hi-fived a guy on Carson who was trying to hale a cab. He moved his hand at the last second.