[_Microft]

There's a tax credit of $7500 that will begin to be phased out as soon as they sold 200 000 cars to US customers.

Phase out starts with the same tax credit still applying for the remainder of the current quarter and the next one (before being reduced in later ones) and that's exactly the reason why they would want to match reaching the threshold to the beginning of a new quarter. This way they have as much time as possible to ship as many cars as possible to their customers (that's OK then since numbers sold do not affect the phase out beside the threshold of 200 000 cars).