I would say that this actually does something remarkable to the statistic. Every child born with Down Syndrome must be highly valued and wanted, and their parents are likely to be strongly pro-life and invested in their human dignity. Any child who was likely to be considered a burden or troublesome, simply didn't make it.
So, you end up with a self-selecting population where the children really have been formed with an optimistic outlook on life, and their parents really did foster environments where they can live happily, and be nurtured despite the disability and obstacles that come up.
It is very nearly a talking point in favor of abortion: that if these babies are truly wanted and loved, they are not being born into a life of abuse, neglect, or disadvantage, which would be a risk for the ones who never made it at all.
Unfortunately, that's not entirely accurate.
50-90% of babies with diagnosed Down Syndrome are aborted before having a chance to be born and enjoy their lives. In Iceland, that figure is 100%.