Why do people respond "it's not your fault" when I say I'm sorry?

A friend told me she wasn't feeling well and I responded with "I'm sorry" to which she said it wasn't my fault. This is not the first time it happened to me. I find that response confusing. Am I not allowed to express my feeling bad another person feels bad without being responsible? or is it more of them saying my sentiment is lost on them?

I know I am probably over thinking it but that response does bug me, maybe its one of those things that bugs me.
Why do people respond "it's not your fault" when I say I'm sorry?
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