Actually I have no idea why the white set takes longer. I suspect that the casing manufacturers have limited production capacity, and due to most demand being the black version, they can barely accomodate the demands for just the black one, and probably are now in the midst of increasing their factories' production lines and machineries to cater for the white ones. But that is purely my own speculation haha.
I initially omitted the Maxis Free SMS from the calculation as I thought that most people would not be able to give an accurate estimation or would be confused to what it means and cause more calculation deviations. But come to think of it, you are right, it does make a difference, especially to heavy SMS users. I've added this parameter into the iPhone Plan Calculator, you can try it out now. I'm defaulting to the assumption of 50% non-Maxis SMS as that is roughly the market share of other networks.
As for data bandwidth, I did not include it as I believe even the minimum offer of 1Gb is good enough for most users - that is 33Mb/day, which is pretty sufficient for internet surfing considering that most people won't do heavy downloads on the phone (i.e. videos and songs are mostly downloaded through PC, then transferred over). Moreover, 3G speed is not really that fast, and it'd be more efficient and economical for them to turn on Wifi should they wish to do larger downloads or faster browsing. Nowadays many places have free wifi available as well, so people get to save on 3G usage. Also, most (especially new) users would not be able to give a good estimation of their bandwidth usage, so they are unlikely to give any meaningful figures that contribute to the calculation.
Thanks for your insightful feedback, Adrian!
