Output of 'last reboot'

Question

Please explain my version of output:

last reboot -F
reboot   system boot  3.10.0-514.26.2. Wed Oct  2 10:45:54 2019 - Thu Oct  3 12:55:56 2019 (1+02:10)
reboot   system boot  3.10.0-514.26.2. Wed May 23 14:35:36 2018 - Thu Oct  3 12:55:56 2019 (497+22:20)
reboot   system boot  3.10.0-514.26.2. Fri Sep  1 08:54:49 2017 - Wed May 23 14:34:51 2018 (264+05:40)
reboot   system boot  3.10.0-514.21.1. Mon Aug 28 12:21:25 2017 - Wed May 23 14:34:51 2018 (268+02:13)
reboot   system boot  3.10.0-514.21.1. Mon Aug 28 12:06:25 2017 - Mon Aug 28 12:21:11 2017  (00:14)
reboot   system boot  3.10.0-514.21.1. Mon Aug 28 12:05:54 2017 - Mon Aug 28 12:06:06 2017  (00:00)
reboot   system boot  3.10.0-514.21.1. Fri Jun 23 08:46:34 2017 - Fri Jun 23 08:48:02 2017  (00:01)
reboot   system boot  3.10.0-514.21.1. Tue Jun 20 17:19:13 2017 - Fri Jun 23 08:48:02 2017 (2+15:28)
reboot   system boot  3.10.0-514.el7.x Tue Jun 20 16:54:11 2017 - Tue Jun 20 17:19:00 2017  (00:24)

What these strings mean?

reboot   system boot  3.10.0-514.26.2. Fri Sep  1 08:54:49 2017 - Wed May 23 14:34:51 2018 (264+05:40)
reboot   system boot  3.10.0-514.21.1. Mon Aug 28 12:21:25 2017 - Wed May 23 14:34:51 2018 (268+02:13)

System booted on Mon Aug 28 2017 at 12:21:25 with kernel 3.10.0-514.21.1? Ok, suppose. But what happened on Fri Sep 1 2017 at 08:54:49 2017? Someone loaded another kernel 3.10.0-514.26.2.? But there was no reboot... please explain.

PS Well, now I am not understanding at all

# last reboot -F

reboot   system boot  3.10.0-514.26.2. Wed Oct  2 10:45:54 2019 - Thu Oct  3 14:50:55 2019 (1+04:05)
reboot   system boot  3.10.0-514.26.2. Wed May 23 14:35:36 2018 - Thu Oct  3 14:50:55 2019 (498+00:15)

after 15 mins

# last reboot -F

reboot   system boot  3.10.0-514.26.2. Wed Oct  2 10:45:54 2019 - Thu Oct  3 15:06:29 2019 (1+04:20)
reboot   system boot  3.10.0-514.26.2. Wed May 23 14:35:36 2018 - Thu Oct  3 15:06:29 2019 (498+00:30)

Why string #2 changed? Server was rebooted by reset button last time (Oct 2)

Answer 1

This looks to me like the server was patched on Friday Sep 1 and this included a kernel upgrade, after which it would be normal to reboot in order to load the new kernel.

Do you have any automated patching? If so then the reboot may have also been automatic, which would explain why you are not aware of a reboot.

(I would have added this as a comment, but I don't have enough reputation points yet)