AMP cache URLs have been showing up as coming from google.com/amp/ - but soon they'll be coming from cdn.amproject.org instead.
As The SEM Post reports, this could be because hackers and scammers have been abusing those cache URLs to masquerade as Google in fake emails. There are other reasons Google gives, too, such as being "human-readable," and "closely resembling the publisher's own domain."
Makes sense! It also means that if you have AMP pages, you should start looking at your backlinks and making sure that you'll be able to keep your link juice long after the change has been made! Google has pledged to keep the juice flowing themselves for a while...but there's no way to know how long they'll keep that up.