Most manufacturers, albeit not all, hence the confusion, give only their minimal response time...
Possibility is that transition times for other colour values are quite different.
Images below show: (images taken from tomshardware, it's to make a point)
As you can see the green line is for a TV that boast a 16ms response time. As it shows it obtains that response time, but only at white-black-white. For any other transition, it has a much bigger value.
The blue line is a screen that puts 15ms in it's specs. Based on the specs you'd say 16 and 15, that's no difference. But as you can see it can be a big difference. (The blue line being much better, cause it has a more constant response time throughout all the transitions). (BTW, the blue line is from a computer screen ;-))