So, nice study, but two things:
1) it may be that when there is no money in it, the finder thinks "by the time we get it back to them, they will have replace their ID and called their credit card companies to cancel their credit cards and issue new ones, so it doesn't really matter". When there is money, it is more likely to actually matter.
2) It may be that the expected consequences of keeping it seem negligible when there is no money, but if you kept it and got found out, when there IS money, then you could be in trouble
3) What is Mexico's deal? The only nation which went the opposite way. Or, perhaps, what happened with the data entry in Mexico that they got the numbers reversed?