The most likely reason is that there were more pre-orders than actual shipments, so they had to delay it to keep up with the demand (which they underestimated). The ironing of the software isn't the reason for the delay, it's just something that could easily be done while doing the main task: to produce more units.

At least this is my guess, but I've to agree about that Nokia's delays suck, as they always seem to happen, yet no one ever has any information about for how long and why it was made, and in many cases the reason for the delay can't be spotted on the final devices (like the case of the N97).