Answer: Altitude fluctuations
The answer is simple, Steve has been given a block altitude clearance from FL450 to FL470 (45,000' - 47,000'), allowing him to fly with the up and down drafts freely. He can also find the most favorable winds within that block, without the need to amend his clearance.
Also, the altitude readouts that have been posted on the blogs are from differing sources at different times. The majority of the altitudes are provided by the "Blue Sky Network", which is GPS based. Some of the blogs as well as the ticker on the main web page have included the "pressure altitude" from Steve's flight data. These two systems have some slight variance between them.
Keep the questions coming!