To your point, there are fights that are so close that they are basically subjective.
Examples:
Moraes-Aldo (Media scores 9-9, no consensus)
Yoel-Costa (media scores 10-10, no consensus)
Cannot possibly be robbery in either direction due to lack of consensus.
However, in my estimation and just my opinion, a fight can be close, but be clear.
Examples:
Jones-Reyes (Media scores 14-7 in favor of Reyes)
Barboza-Ige (Media-scores 15-2 in favor of Barboza)
Both of those were close fights. However, I personally consider them robberies based on the following.
There was consensus regarding the winner among fans and media scores. The ringside judges are in the far minority. Consequently, the only thing keep them from their wins were minority opinions of the ringside judges (who notoriously suck).
That is my definition. If the vast majority of people understand that fighter X won, but the judges have a very unusual opinion that fighter Y won, I consider it a robbery, close or not. just my opinion.