Accepted by European university, rejected by all American ones I applied to? Possible reasons? I've been wondering this for a while.

I'm currently doing the first year of my PhD in Frankfurt. Originally from New Zealand.

I'm not complaining about my situation but if anyone asks me I'll honestly say I would have preferred an English-speaking country.

I'd actually applied to about 9 PhD programs in the US and all rejected me. Yet I was accepted to a (supposedly far more competitive) fully funded European PhD, and rather quickly - accepted late last year and already working now.

It has me wonder. I had very strong letters of recommendation from professors that are well known in my field. I had also published a paper (which has now been cited in Nature) out of the fruits of my research Master's.

My GPA is about 2.8-2.9 by US standards (hard to convert, my school was a "we grade harshly, rarely give out As and we'll make you sweat for a B" style system). Professors at home told me my grades were irrelevant with the strong letters.

People say that I was likely discounted because I had a research master's already and I'd probably see doing more course load as beneath me? That doesn't seem to hold water.

I really don't know. Any thoughts?
It is sometimes difficult for admissions committees to evaluate applicants coming from universities or university systems that they don't have direct experience with.  This is especially true if they also don't know your letter writers.  It can help to apply to universities where someone in the dept. has some connection to the country you're applying from.  Since someone at Frankfurt knew one of your letter writers, that person may be better informed about your letter writers, the quality of your school, and the meaning of your grades.