I assume (but have not verified) that these files are overwritten (which the drive firmware may map to different sectors) whenever the kernel images' post install trigger fires, i.e. on kernel updates.
It could also be that this only occurs on grub-install, which I may have run around this time on this system.
A 16.04 system I'm looking at shows current modification times on all files in /EFI/ubuntu/ files:
# ls -l /boot/efi/ EFI/ubuntu/
total 3484
drwx------ 2 root root 4096 Apr 30 2017 fw
-rwx------ 1 root root 67536 Jan 26 18:07 fwupx64.efi
-rwx------ 1 root root 152 Jul 10 10:40 grub.cfg
-rwx------ 1 root root 1133944 Jul 10 10:40 grubx64.efi
-rwx------ 1 root root 1153336 Jul 10 10:40 mmx64.efi
-rwx------ 1 root root 1196736 Jul 10 10:40 shimx64.efi
I assume (but have not verified) that these files are overwritten (which the drive firmware may map to different sectors) whenever the kernel images' post install trigger fires, i.e. on kernel updates.
It could also be that this only occurs on grub-install, which I may have run around this time on this system.