High Sierra beta (10.13.5 beta 2) kernel variable changes

Is it normal for the kernel variable kern.boottime to change between startup and shutdown. I have a script, run as root, at startup time, by launchd which records kern.boottime in a log file sysctl -n kern.boottime >>/usr/local/data/boot.log If I execute the same command, in Terminal, after logging in a few minutes later, the value has changed (now indicates 3 seconds earlier.) I have done this several times in the last couple of macOS releases and the same 3-second difference is noted. Does this sound like a bug, or purposeful? Thank you in advance.