The cause is two-fold:
1.) The "Importance markers" in Gmail are not enabled by default, so it may not be obvious which email is marked as important.
2.) By default, Gmail has it's own algorithm to predict and determine which emails are marked as important.
The following Google KB article provides more detail:
You can use the section titled, "Hide importance markers in Gmail" to "Show Markers". You can also use the search string "is:important" within the Gmail Inbox to find items marked as important.
© ALL RIGHTS RESERVED. Feedback Terms of Use Privacy Cookie Preference Center