Apparently each SPWeb can have it's own list of TimeZones (assuming that is why there is a TimeZones property on each SPWeb's RegionalSettings).
Why could/would a web/site within a site collection/farm have different collection of TimeZones (i know why a different web/site would have a different TimeZone selected, I am asking about the "collection" of time zones - i.e. the list of values that populate the TimeZone dropdown in the /_layouts/regionalsetng.aspx page)? Wouldn't the possible list of selectable values be the same for each web across a farm?