Why does synID() always return zero when inserting at the end of a line?

I ran into difficulty trying to automatically format C-style comments without a middle part. I was trying to set formatoptions automatically depending on whether the cursor was inside a comment or not.

My problem was that when in insert mode at the end of a line (e.g. after using 'A'), synID() always returns zero, even if the comment region extends onto the next line.

I worked around the issue by using col('.')-1 instead of just col('.') if I'm in insert mode...but why do I need to do that?

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