Halliday & Matthiessen (2014: 368):
Note that there are two different types of some, one selective and one non-selective. The selective some contrasts with any, and is pronounced some [s^m]. The non-selective some is reduced in spoken form, being pronounced [sm], with a syllabic m ; the limiting case of phonological reduction is absence, and [sm] does indeed alternate with the absence of a determiner (Ø). Thus in George Orwell’s All animals are equal but some animals are more equal than others, the determiner all is a positive total Deictic, whereas the determiner some [s^m] is a selective partial Deictic.