Why Nikon D60 then D7000, then D90?

Why is Nikon releasing funny sequence D60 then D90 then D7000 then D2100,… Why is this unstable numbering… Anyone can explain please?

Added (1). I tend to disagree with the Jen's answer simply because big digit models began to come before the D60 and D90 hit the market. We need the real story to avoid making a wrong choice. I know some brands use two parallel releases one w high content of plastics and the other is of titanium or some tough materials; keeping the engine same.

The number of the digits is supposed to indicate the customer segment that the camera aims at. Apparently, since the two digit product numbers have reached the 90, there's a change in this numbering scheme though. The two digit numbers were used inconsistently as well too, for both entry level and prosumer cameras.

So now it basically work so that the four digit numbers are the lower segment, the three digit numbers the intermediate segment and the single digit numbers the premium segment. Lower segment is to be understood very liberally here though, since the D7000 is quite a powerful camera.

Within a number-of-digits class, higher values still mean a higher ranking of the camera.

So the current lineup looks like this:
D3000 - low entry level model
D3100 - replacement of the former low entry level model
D5000 - high entry level model
D7000 - prosumer camera
D300s - high end APS-C camera
D700 - low end full frame camera (but higher than high end APS-C)
D3X/D3S - high end everything

It is NOT really "unstable" numbering. What has happened is that the Dxx series was filled up and a new series had to be started.

Why for instance did Canon make it full frame cameras 1D and 5D and then, out of the blue make the 7D a cropped sensor camera?

Each company has its reasons for the numbering systems. What should concern you is the performance of the various cameras