Most likely because of observing something similar as you are seeing right now. I can no longer remember the details exactly.
As with so many things, it’s a complicated process. If an OS boots up, it doesn’t know about time. On PC hardware the (battery buffered) RTC is then used to derive of a first sample of time. While Windows defaults to local time here, all the UNIX and LINUX systems I’ve used so far default to UTC (at least if no Windows is detected on the same machine). Wrong time can be detected and corrected later on as soon as the network is ready and internet access available by using NTP do derive an independent time sample. UTC is usually set as the default TZ on UNIX/LINUX systems. As ROCK doesn’t allow a user to make changes to the system and AFAIK doesn’t allow to set the TZ in it’s web UI also, i would therefore assume it’s (still) UTC. See also: