How standardized is lab-grade skim milk?
There is a degree of variability in milk, and in some cases this cause problems in protein work. Despite being a standard for ELISAs, it's not known to be the most reliable blocking buffer. As far as cryoprotection goes, variability doesn't seem to be a major concern.
Standardization varies depending on manufacturer. The Sigma link you included reports a quality level of MQ 200. The MQ level is something Sigma created in an attempt to increase clarity about quality controls. Products rated MQ 200 are for non-regulated applications. For this level they will provide a site-quality self-assessment, a certificate of quality/analysis, and specifications. They also have a notification system that customers can opt-in to for updates on quality control features. There's information on what each quality level meansand a breakdown of what they will notify you about on the Sigma website.