Showing a bump - why do they say 20 weeks???

Why do they say you don't show until about 15 - 20 weeks usually? Does that mean showing enough where people will notice? I'm 10.5 weeks and I definitely have a little bump which is getting hard to hide at work (especially by the end of the day! I feel 5 months pregnant then...). Why do they say 20 weeks usually until you show (with your first baby) when people seem to start showing right away often... Especially first from bloat and then from a bump! Is this still just bloat right now for me at 10.5 weeks and the bump hasn't started? I'm confused... :)