I think lots of people find them to be inaccurate... not terribly so, but inaccurate none-the-less.
I was thinking about this last night as I'm considering taking one to see if I'm 'progressing' to, and my thought was this: does it indicate (accurately or not) from conception, or implantation? I would assume implanation as once that's completed (in and of itself can take about fours days) is when the hcg starts filtering into our systems. Obviously, this can make quite a big difference to the indicator, especially for someone who perhaps ovulated a little late in their cycle and also implanted late.
I know my O date as I was charting (five dyas PAST the 'average'), and I know implantation must have begun by 6 days past O as I had some spotting... but most women wouldn't have that information.
I would say that, in theory, you can get the 2-3 weeks result for two weeks entirely - have you passed that mark?
And of course, you could always speak to your GP about an early reassurance scan given your history - I know I would in your shoes.
Hopefully everything's just fine - there are quite a few variable with these tests - and I look forward to hearin good news from you!