I have not heard this actually but here is what the person you heard this from may have been trying to get at.
Now this is just a rought example but when scouts are looking at pitchers we all know they look at a few different things but as far as velocity goes for a right hander they would like you to throw the ball 90+ with a few exceptions and for lefties the number is more about 87-88. This is simply because there are fewer lefties at the next level and it is believed that by this reason they are harder to hit.
Also whether is actually true or not there some people in baseball who believe lefties have more natural sink and or movement on their pitches. I honestly have no clue if anyone has actually tried to prove this but who knows.
Basically because lefties are so sought after, if you can find a lefty who throws strikes, has control of his offspeed and can run it up into the 90's he will be a top pick, it is much harder for a righty.