speachless: 'Stronger' relates exclusively to the current tournament, and the number of wins the player had in it. So the SB is the sum of the points the opponents you beat had (plus half the drawn opponents). For example, if you have one point because you beat a player that has 4 points, and I have one point because I beat a player that has 5 points, SB considers me ahead - as I beat the 'stronger' player.
Remember that SB comes from live (chess) tournaments, where there is not neccesarily a BKR or any other rating available - people might have never played before publicly, or they might have multiple ratings in different systems.

It is an interesting idea though, to use BKR ratings instead of wins in the current tournament...