What changes ModifyDate in jpg metadata?

[gmit2]

Looking at a Samsung phone jpg photo I see an interesting pattern that is puzzling.  Hour:minute:second is the same despite ModifyDate being more recent than DateTimeOriginal and CreateDate.
   DateTimeOriginal and CreateDate = 2018:01:02 12:13:14
   ModifyDate                                 = 2019:02:03  12:13:14
This pattern is repeated for a different jpg photo from the same Samsung cellphone.

I am suspicious because the time is the same on ModifyDate as DateTimeOriginal and CreateDate.  2 questions:

1.  Could it be that something triggers ModifyDate changes that affect the year:month:day but desn't change the hour:minute:second?  I suspect that this is the case.

2.  What would be expected to change the ModifyDate?  Just a change of the photo content?  Perhaps something more subtle related to file access.

[Phil Harvey]

1.  Could it be that something triggers ModifyDate changes that affect the year:month:day but desn't change the hour:minute:second?  I suspect that this is the case.

Other than this specific command:

exiftool "-modifydate+=1:0 0" FILE

I can't think of anything that would do this.

2.  What would be expected to change the ModifyDate?  Just a change of the photo content?  Perhaps something more subtle related to file access.

It should be only if you change the photo content.

- Phil

[gmit2]

Further confusion.  I modified the file before.jpeg with an editor to add a shape saving the result as after.jpeg.  The ModifyDate did not change despite the files being different sizes. I tested this by:

On a Mac looking at the before.jpeg and after.jpeg:
exiftool *.jpeg | grep "Modify"
gave the same (old) Modify Date for both files

On Windows 10:
Exiftool *.jpeg | findstr "Modify"
gave the same results as on the Mac

Looking at the files with ls or dir (as expected) shows that the file sizes are different.

Hypothesis #3, the ModifyDate tag is only changed by the phone, not a computer that received a file from the phone.

[StarGeek]

exiftool *.jpeg | grep "Modify"

I'd suggest adding -g1 -a to the command at the very least.  That way you will get all tags even if there are duplicate names.  Also, you don't need to grep the answer.  You can use wildcard in the tag name.
exiftool -g1 -a -*modify* *.jpeg

ModifyDate isn't automatically changed if the image is edited.  It will only change if the program that edits the image changes it.  There are a lot of lazy programmers out there that won't bother to do this.