An American is staying in my house for the summer and claims that food (specifically, bread and milk) spoils a lot faster here than at home in California. Is there a significant difference in chemical content (preservatives etc.), or other factors, that might explain this?
I would think added preservatives are the most likely candidate, for example, Azodicarbonamide is frequently added to bread in the US, but banned in Ireland and most of the rest of the EU. Since it has a preservative effect it is a reasonable candidate for a specific causal factor.
But as Keegan points out there are an awful lot of possible factors so it's impossible for us to say with any certainty.
There are more factors than we can ever consider.
As far as region: does California represent all of the US? What part of California? What part of Ireland? What's the humidity and average temperature of each region? What are the agricultural regulations?
As far as cultural/methodical differences: how do you store the food in your homes? The quality of your cooling method? The way people in the household handle the food?