Timer skew

I am testing ULPM on Mangoh Red board and have it working.
The ulpm wake timer is set to 55 min (i.e., 3300 sec)
In order to determine “boot complete”, an autostart oneshot timer with period of 1 sec is used.
When the boot timer value is 1, the assumption is that the device has completed booting and
is ready to handle the application actions (i.e. like set and fire timers).
[Please suggest a better mechanism to use to determine “boot complete” if one is available.]
When the boot timer fires, a second “ulpm_up_time” timer is set to 5 minutes (i.e. 300 sec).
This is a oneshot, not autostarted timer.
This timer is used to determine the duration of the wake periods between ULPM re-entry.
When the ulpm_up_time timer expires, ulpm is started.

So the sequence executes ads follows: the device boots, the boot timer fires once, resulting in the ulpm_up_time timer getting started, and running for 5 minutes. When it expires the module enters ULPM which lasts for 55 minutes. When it wakes, the same sequence is executed.

I expected to see some skewing to the right (i.e. later) in the times due to the unknown time involved in the actual boot (i.e. wake from ULPM). However the times I am seeing are actually a negative skew (earlier) by about 40 seconds. I cannot figure out how it is possible to skew the times to the left (earlier) if the 55 min and 5 min timers are are not overlapping.

I have attached below the power up times from the recent device test. Can you please explain how
this is possible? Can you advise the best way to obtain a consistent time needed to wake and report data?

Apr 29, 2023 18:33:08 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 17:33:49 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 16:34:30 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 15:35:11 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 14:35:53 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 13:36:32 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 12:37:13 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 11:37:53 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 10:38:33 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 09:39:15 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 08:39:53 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 07:40:32 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 06:41:10 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 05:41:49 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 04:42:28 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 03:43:08 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 02:43:46 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 01:44:26 GMT-7 -- {"action":"POWER_ON"
Apr 29, 2023 00:45:05 GMT-7 -- {"action":"POWER_ON"
Apr 28, 2023 23:45:46 GMT-7 -- {"action":"POWER_ON"
Apr 28, 2023 22:46:24 GMT-7 -- {"action":"POWER_ON"
Apr 28, 2023 21:47:04 GMT-7 -- {"action":"POWER_ON"

Thanks