It is true, because some is not an absolute term. The distinction between absolute and qualified terms is about the purposes, where absolute shows limitations and restraint for the given topic. For example, The stars can only be seen at nights - It is an example of absolute quality. We have a strict restraint here that the stars can only be seen at night time. But Some frog species only have one leg is an example of qualified term - In this sentence we show that there are other species, as well which we are not discussing.