That's because it's a nasty jail.

Over here in Finland prisons are about brainwashing, not about nastiness. They are relatively comfortable places, although being cut from your usual social circles does have its toll on the mind.

And we have among the lowest crime repeat rates in the world. The American penal system is built for retribution, accepting that that means more crime.

In Finland, and to a very little lower extent in the Netherlands, the penal system is primarily about stopping people from committing a crime again, even if that sometimes results in convictions that feel unfair to the victim.

When you leave a jail in USA, you have been conditioned into a very criminal way of thinking by the social conditions of the jail, and you are more likely to do crime than you were before the jail. And that's fine because the point there is not to reduce crime but to have retribution for victims. Jails work extremely differently on this side of the pond, because they are about crime prevention.

15 monrhs in a Dutch jail do indeed have an effect.

And in any case: you talked about the effect of jail experience on repeating an offence. There's, however, also the effect of the threat of jailtime generally preventing crimes. This has been researched in criminology and once the punishment is strong enough to hurt, increasing the punishment has a negligible preventive effect. A month in jail is such a strong punishment for smoking weed that if you're smoking it anyway, you're already doing that in the belief that you are able to completely avoid being caught. If you felt there's a real risk for a month in jail, you wouldn't smoke the joint at all. If the punishment was one day of prison, you might take the risk, but when it's a month and you still commit the "heinous crime", it shows you assume you cannot get caught.

And for those people it doesn't matter whether the punishment for the crime is one month or two years of prison. Those who are to be deterred by the jailtime are deterred already at the lowest effective amount of jailtime, the rest won't really change their behaviour because they assume they're skilled enough to not be caught in the first place. The increased jail time does have an increased retributive effect, although I'm not sure who is getting the retribution in case of weed smoking... But I'm not living in USA, so I don't really need to care of their logic there.