Alternative timelines

There's a subgenre of a subgenre, known as historical fantasy, in some part of which, as the name suggests, history gets tweaked. It often gets thrown in with speculative fiction, usually associated with sci-fi, fantasy and horror, but where do you draw the line? As a fairly obvious example, Stephen Fry's Making History used a scenario in which (if I remember correctly) Adolf Hitler had never been born, but I don't recall seeing that novel hidden away among the edgy experimental works next to the fantasy epics in seventeen parts.

All fiction is in some respect speculative, otherwise it would be biography - at the very least there is a mainstream character whose actions and reactions in certain scenarios are imagined and plotted. Often a novelist places their characters in the middle of 'historic' events, interacting with characterisations of real people, having conversations with the rich and famous that never took place - like Forrest Gump. On a smaller scale, the character is said to be in Northumbria, goes to the market in Hexham, but the village in which they live is entirely fictional; they attend a college at Oxford that never existed, shop in an imaginary London department store on a real street opposite real shops, and buy made-up magazines at WHSmith.

I can understand wanting to distance your characters from the real world slightly; readers are a pedantic bunch and they will notice if a description of a real village has the war memorial on the wrong side of the bridge. What I can't always understand is the need to subclassify, as though books were a kind of organism with hundreds of different species. There are a few genres I shy away from (chick lit, romance, that kind of thing) but I've been misled by genre labels before. 'Classics' to me meant Bronte and Dickens: syrupy, heavy-going and dull. Persuaded by my dad about ten years since to give Anthony Trollope's The Warden a go, despite the fact that Trollope sits well within that category, I was surprised by how much I enjoyed it, and if you're a regular reader of these ramblings you'll know what a fan of Trollope I am now. There are people who've read and enjoyed Gormenghast but wouldn't touch a fantasy novel with a bargepole, or consider 1984 a classic but wouldn't consider sharing in Philip K Dick's dystopian visions.

Part of the subclassification is down to laziness, I suspect - a busy reader doesn't want to waste time wading through thrillers, violent realism or weighty Victorian sermonising when what they're looking for is happy ever after. Personally I'd prefer it if all fiction in libraries and bookshops was ranged alphabetically as a whole and I was left to make up my own mind.