Those famous battle markers are part of the RTW.exe's hardcoding. They don't always make sense. Sometime a huge battle that is difficult but won against all odds won't get a marker but an easily won victory will get a marker.

The time the last seems odd too. Sometimes they'll seem permenant and sometimes they'll disappear after a decade. I think the time they last is tied to turns and not years so they disappear faster in EB than in RTW if you are counting years. The time they last does seem to be tied to the size of the battle, though.

Since it is part of the exe hardcoding, nobody can be for sure on the details.