May be a dumb question...

Not trying to offend anyone so sorry if I do!! But how come people are getting super faint positives when they are already 5+ days late on their period?? This confuses the hell outta me. My test was super dark, pulling color from the control line when I was 6 days late(I was about 5 weeks pregnant by then). So I'm just wondering why some women's hcg levels are so low even after a missed period??