Alexa is so unpredictable and not realistic at times..it can give you a general idea of whether a site is pumping traffic or not, but not to any type of exactness, since it is only counting traffic from surfers that use their toolbar.
For instance, right now my site is at 113k but traffic is up 10 times greater than when it was ranked around 85k ....it all depends on the rankings of ALL sites and how many people are surfing with their toolbar
Ivy