When it comes to bikes and frames there literally are no rules. I've got five bikes, seven if you count the tandems, and seat tubes and effective seat tubes are all over the map. Effective top tubes are all about 22.5". I've seen published geometry tables that use the same ETT for all the seat tube lengths except maybe the very shortest and the very longest. The Giordano Libero comes to mind as a particular example of that kind of design. I get from this that the rider is expected to use stem length and/or handlebar reach to dial in the amount of 'stretch' that feels right to them. When I'm shopping for a bike, its dimensions are not the criteria that concern me most. I can usually make most anything I find fit me, and fairly inexpensively at that, by using the right stem and bars.
H