You benefit from an equipment loot bonus of 155%. This means that there is a 155% chance for you to receive another set of equipment loot from the boss.
This bonus only applies to items which can be equipped unless they are limited to one drop per boss kill, e.g. Ferumbras' hat. Such unique drops are not affected by this bonus.
Using the example of The Welter, I suspect that the rare items (necklace and crossbow) it drops are unique, as I have never seen more than one person drop the same rare item. So it is very possible that the boss bonus does not apply to these items. Unfortunately, CipSoft has never wanted to release an official list of the equipment they consider unique, so it is up to players to draw their own conclusions.
In addition, the boss bonus message only appears if you actually drop some equipment in the extra set of equipment loot. In your case, the items you dropped (knight armor, ice rapier and stone skin amulet) came in the boss's normal loot, and not thanks to your 155% loot bonus. This means that regardless of your amount of boss bonus, it is not guaranteed that the boss will drop equipment in the extra set of equipment loot.
Source:
https://www.tibia.com/news/?subtopic=newsarchive&id=6733