@Greenberet seems to have the best answer. I also found RFC 6598. https://datatracker.ietf.org/doc/html/rfc6598 "This document requests the allocation of an IPv4 /10 address block to be used as Shared Address Space to accommodate the needs of Carrier- Grade NAT (CGN) devices ... The Shared Address Space address range is 100.64.0.0/10." I'll guess ChromeCast has some kind of internal address, or perhaps even its LAN IP, and is wanting to NAT it into a known address space, so they chose this CGNAT block.
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