tempus.edax.rerum, let's make it even worse: what is "consciousness"?
There is indeed a problem:
When you do such tests you aren't sure where to draw the line between "know" and "not exactly". Comparing your results to results of other people is complicated. You have to hypothesize about how they draw the line.
But there is only a problem when you look at it as a "test". It is not a test.
If you're for some reason are curious about this, if you want to know how many words you know, you just define what is "know" for yourself (approximately). Then you can use such "tests" as a tool for estimating the size your (and only your) vocabulary. Approximately. Under varying definitions of "know", "size" and "vocabulary".
Given that you already want to know how many words you know, you likely already have some idea of what the word "know" means:)