Why is a “seen time” on web in messages different from the “seen time” in..?

Hello,
I've announced, that my ex, which I am trying to get back has read my message in certain time in the morning. That time was initially the same in both (on web and on the FB mobile app). But later I've announced that time has changed two times on mobile app. It remains the same on web. Can somebody explain?
Thank you

See original post by Davey3