The ‘automatic’ tactical adjustment
The tactical adjustment devoiced /z/ to /s/ after voiceless obstruents. (The explanation for obstruents you can find under “The two Middle English Changes”.) Very often the final voiced obstruents, those ones in the end of a word, were less voiced, than initial (in the word beginning) or intervocalic ones (those ones between two vocals). It might be that they devoiced so much, that it was almost impossible to differentiate between voiced obstruents that seemed to be voiceless and those ones who were in fact voiceless.