This web site redgoldfish looks very busy all the time. Have a look and you will see more or less 2000 users at anytime. Now, In alexa it's ranked 246,031! My web site when it gets just +150 unique user a day I could see the daily rank goes down to 95.000 - even less! Now, go to redgoldfish and reload/ refresh the page and you could see the number increments at every refresh! Does this make sense?
Forget about Alexa as a measure of traffic...it's is a uselss estimate unless you're in the top 500. All Web Dev, directorties, and Tech related sites will always have a higher Alexa score because the majority of visitors are using their toolbar. For example, I am getting about 4,000 - 4,500 uniques/day (up from ~ 3,500 last month) and my Alexa is still above 200,000.
Alexa rankings seem to fluctuate a lot, so I won't count on them on being that reliable just yet. ~ Sirius Lin