How can the output of `uname` change in a VM after a physical server was patched?

My Zabbix monitoring system told me that the uname output has changed on a virtual server (Host information was changed on ...). The ISP has shut down the entire server in order to perform some security patches on the physical hardware. How can it be, that the kernel information, which is received by the uname syscall, has changed after that? In my understanding, the patches pertain a completely different system layer and should have left the software OS VM layer untouched, isn't that so?