how many Yahoo databases are there?

[quantobasta]

After noticing that the number of serps Yahoo returned when I searched site:mydomain.com varied through the day I figured I must be accessing different databases. I then did a ping of search.yahoo.com after each search and received different ips for different result sets. Here are the two I have seen so far.

search-east-vip2.yahoo.com 216.109.117.133
m3.search.vip.scd.yahoo.com 66.218.71.233

At first I thought it might be a matter of propagation like on Google with www2.google.com being changed prior to www.google.com. However, if this was true, I would think than one of the Yahoo result sets would be a subset of the other. This is not the case. Each set has SERPS not found in the other. If I search site:mydomain.com mydomain I get a much larger set of serps regardless of the Yahoo ip being used.

I have read posts speculating that Yahoo serves different results depending on the location of the searcher so for what it is worth I am in the SF Bay Area.

Any additional information to flesh what is going on here will be appreciated.