Because the highlight text colour has changed from white to light blue and the build number has incremented by three.
When you get down to the 4th level of the build version this is very small/minor a change to the 3rd level more so and obviously the 2nd lever is in line with a regular program update (note the 3rd level would also change in this instance.
An example of the new colour is the Firewall tab, where "Firewall options" text is now light blue in colour.
edit: Forgot to add the screen copy.
I'm not aware of any colour coding to indicate an emergency program update, but then again I'm still using Avast Free and version 20.2.2401 (build 20.2.5130.570).
I may even wait for 20.4 before updating as I didn't see anything critical or worth updating in the 20.3 change log. But then again the change logs have been pretty abysmal with what's changed/fixed/updated, lacking in detail. You almost need to view the beta builds to get any idea what has been fixed, known issues, etc. etc. and I don't see any of these feature in the 20.3 program release notes/change log.