The Serial.print() and Serial.write() methods in 1.0 now use interrupts to send serial data. The data is stored in a buffer, and the Serial.print()/Serial.write() commands return immediately - unless the outgoing buffer is full. If it is, the functions block until there is room in the buffer for the data they are trying to place in the buffer.
To do a real timing test, you need to call Serial.flush() after buffering the packet. Serial.flush() does nothing but block until the buffer is empty.
That way, every packet send time is consistent. Otherwise, some packets fit in the buffer, and some do not. Hence, some packets cause blocking, and some do not. In which case, you will not get consistent times.