They're really only accurate in telling you how much HCG is in your urine. My blood HCG was 39 on 11 dpo (3 weeks 3 days), and I got "2-3 weeks" (past ovulation) on a weeks estimator at 14 DPO (3 week 6 days). Then at 4 weeks, 1 day my blood HCG was 520 and at 4 week 6 days I got "3+" week past ovulation on a weeks estimator.
Essentially they're good for letting you kbow your HCG levels are rising. The levels of HCG in your urine are also a little more diluted tha blood HCG.
Ugh. Sorry for all my typos. It's my phone.