Well then lets say you could either get a knife in a case or not. If you were to open 1000 cases you could get an estimate (It wouldn't be fully accurate but probably close enough) of how often you get a knife out of a case. For it to be 50/50 you would need to get an equal number of knives as other weapons. Other wise the can determine the chance by comparing how many knives you got vs how many other weapons you got. Most likely you'd get between 2 and 5 so lets use 4. Then the percentage chance would be (4/1000)*100 which is a 0.4% chance. Meaning the ratio would be 0.4/99.6, not 50/50. I know you already know this and I'm not explaining it trying to seem smart or anything, just if you were looking for an explanation that would probably be the best I could come up with on the spot.