Statistics are like bikinis. What they reveal is suggestive and
what they conceal is vital. How one interprets such data is up to
you.
There are numerous possible explanations on this:
1. Atheists know how to behave responsibility in the face of
law.
2. Atheists are better at avoiding cops.
3. Atheists knows the loopholes in law and makes use of them well,
probably due to their questioning nature
4. Catholics think their sins will be forgiven easily and therefore
are less likely to take responsibility for their actions.
5. Catholics confess to their crimes easily, probably due to
life-long conditioning... 
6. Catholics are more likely to succumb to temptations and
therefore needs religious guidance? (don't flame me on this. It's
only speculation)
7. Catholism is more of a family religion, passed from generations
to generations. Choice of catholism might not be conscious, whereas
atheists might take a more active role in their beliefs. This
active behaviour is reflected in their conscious avoidance of
illegal behaviour.
As you can see, tons of explanations and I can still go on. The
main point illustrated by these figures, is that religion is not correlated to conduct/ lawful
behaviour.
Of course you cannot compare each and every different country
and expect the same explanations for their culture and behaviour.
This I agree.
However, due to lacking of data from Singapore and the strict
control over media, I doubt anyone can get a breakdown by religion
or race of prison inmates for that matter. US may be a christian
nation but the statistics were compared to the population
breakdown. Whether the country is multi-religious or not, does not
matter cos' we're comparing the relative proportions.
Furthermore, what makes Singapore a good country for judgement and
comparison then? It has a conservative government and is not
representative of the world either.
They are not classified automatically. Instead the religion of
each inmate in US were asked prior to imprisonment. As I said
earlier, Catholism is passed from family to family, generation to
generation, and thus people might take a lesser active role in
trying to find their own faith or even attempt to take
responsibility for their actions. This is only speculation and IS
NOT A SWEEPING STEREOTYPICAL STATEMENT.
Now, anymore questions and doubts people? 
