Where are you seeing these uptime values? I suspect you are running a custom report that calculates uptime dynamically.
The uptime value listed in the Summary tab of a computer's webpage is not calculated dynamically, but retrieved from the computer itself when the machine is scanned. Specifically, the WMI command below is run on the client machine. If the machine is offline, there is no way for Lansweeper to retrieve the uptime information and the value retrieved during the last successful computer scan will remain listed. This value will not change until the computer is successfully rescanned.
wmic /namespace:\\root\cimv2 path Win32_PerfFormattedData_PerfOS_System get systemuptime