There is always added 45 or 55 second on the end of the target time. I compared mine to someone else and his was set to go of at that specific time
Because of the combination of being a cloud-based system and a mesh network, nothing is guaranteed to run at the exact second that you specified. Within one minute one way or the other is considered normal.
SmartThings will try to execute your scheduled job at the specified time, but cannot guarantee it will execute at that exact moment. As a general rule of thumb, you should expect that your job will be called within the minute of scheduled execution. For example, if you schedule a job at 5:30:20 (20 seconds past 5:30) to execute in five minutes, we expect it to be executed at some point in the 5:35 minute.
I don’t know, but they may do some load-balancing as well and slightly offshift different accounts by different amounts.
If you need very precise timing, you’ll need to go to a Wi-Fi based system that runs completely locally and not use SmartThings.
Architecturally, I sure hope they do!