Some snapshots not flagging as changed

[immewnity]

I'm tracking an extremely simple page on my own website, consisting of just a single h1 element with text in it. I'll change this text to test if alerting is still working. I noticed that one change didn't show up on the Watching page or send a notification, but still showed up in the "past snapshots" as a change.

Snapshots page:

Watching page ("Heroku Test" is the page in question, previous changes have pushed out of the Watching page):

Slack notifications:

The change from 52002674 to fed483e7 was relatively minor (addition of two words), whereas fed483e7 to 3b621cb9 removed all existing text and added new words. Could the amount of change be related to why alerting failed?