Why do I fit into a 36C when my boobs are far from average?

I'm a 19 year old, I wasn't always chesty but have put on weight, so obviously my chest has increased quite a bit. I'm aware that I have a fairly large chest, but when I was out bra shopping I was shocked a 36C bra fit me. Is it just the fact that each brand has different sizing systems or that I'm really just a girl with average sized boobs?

Why do I fit into a 36C when my boobs are far from average?
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