It's a false comparison. No companies in my round (W07) or before that I'm aware of got anywhere near as low as $1M valuations...
For the $500-800k raised that is still popular, "OK" was a $2M pre valuation and "amazing" would have been $5M.
The $10M valuations today for $500-800k (if there are any) are definitely in the "amazing" range, so it's somewhere around a 2x increase.
That could definitely be explained by a more mature set of companies, or companies attacking larger problems.
Are these the valuations that they're getting from YC, or from investors post-YC? If post-YC, couldn't this be due largely to more of the companies being Ramen Profitable? If they raise money after four months rather than three, they've been growing (perhaps at an exponential rate) for 33% longer.