Why is my Chinese weird?

There's something about my Mandarin that strikes people in Taiwan as odd.  I still can't figure out exactly what it is about the way I speak that they react to.

During my week in Taiwan I got a lot of comments like this:

"Your Chinese sounds very standard." (你國語講得很標準。)
"Your Chinese sounds refined."（你國語講得很文。）

These two comments are quite different from the "Oh, you speak Chinese so well!" variety of comment that all Chinese language learners, regardless of ability, hear frequently from Chinese people.  They are picking up on something unusual about the way I speak.

I was also asked "Did you study in the mainland?"

So at first I thought that these comments must all be due to the fact that my pronunciation includes some features that are more common in the Beijing area than in Taiwanese Mandarin.  For example, I distinguish the retroflex (curled tongue) sounds transcribed zh ch sh from the non-retroflex sounds transcribed z c s.  So in my speech, sān 'three' and shān 'mountain' are distinct; for most Taiwan speakers, they are homophones pronounced sān.  I also have a so-called "neutral" or "unstressed" tone in some syllables where Taiwanese speakers have a full stressed tone.  For example, I say xuésheng 'student', not xuéshēng.  Again, this is more common around Beijing than in Taiwan.

But I asked a few people and they said quite emphatically that I don't sound like I have a mainland Beijing accent.  And in fact I don't.  My Mandarin is something of a hybrid reflecting the various places I've lived and studied, including both Beijing and Taipei.  So something else is going on.  Something about sentence intonation, accent, word choice -- not sure what it is, but it gets me noticed.