What's the difference between 'top servers' and 'active sites' in Netcraft's web server survey?

Question

I'm looking at the graphs on this page and I don't understand the difference between two market share graphs:

• Market Share for Top Servers Across All Domains
• Market Share for Active Sites Across All Domains

I understand the distinction between hostnames and active sites as explained here, but I can't quite grok what exactly the top servers graph is showing. If you look at the stats for MS in March 2002 one graph says 34% and the other 27%.

Answer 1

I would assume that the primary difference is that one is measuring the number of distinct active websites using each web server, and the other is counting the number of servers (machines or virtual machines) running each web server. Often large sites are hosted on multiple servers (or server instances), so even if a web server is hosting fewer active websites, it could still be running on more servers if the sites it's hosting are larger (or simply split their traffic over a larger number of servers).

Answer 2

I emailed Netcraft and they said they're going to rename the graphs to be clearer.

1. Total Sites Across All Domains = Total number of websites
2. Market Share for Top Servers Across All Domains = Web server developers: Market share of all sites
3. Market Share for Active Sites Across All Domains = Web server developers: Market share of active sites
4. Market Share for Top Servers Across the Million Busiest Sites = Web server developers: Market share of the top million busiest sites

So IIUC the difference is that the Market Share for Top Servers Across All Domains includes sites that aren't considered active, whereas Market Share for Active Sites Across All Domains only reports on active sites.