I've found that to be true as well. I can usually get below what the gear is "rated" for because I sleep well in the cold, but I have no problems sleeping comfortably with a AHE 3 season UQ (rated to 30F) and a 1 season Owyhee TQ (50f) into the mid to lower twenties, I actually overheat really quick when I break out my 3 season Owyhee. I should note that I sleep in lightweight polartec pants and shirt, with a fleece sweater and wool beanie cap. I also use a down jacket as a foot box (still thinking about getting a full length UQ). I could probably get a bit lower in temps if I wore heavy weight long underwear.
I think what you wear to sleep in can make a huge difference in how warm you will be, assuming you are properly insulating the underside of your hammock.