Tamriel is clearly an Earth-based planet, as almost all Fantasy worlds are. This approach has been proven to make fantasy worlds more believable, by giving the reader/player something to relate to (our own planet).
As a rule, the geographical landmass of this fantasy world must now relate to the Earth's, taking into account the equator, prevailing winds, weather, ocean currents etc, to create a realistic layout. A quick google image search of 'biomes' will show how these factors affect the geography of our planet, and with fantasy worlds so often having identical climates and planetary positioning to our own (including Tamriel), their own biomes should follow the same guidelines in order for the world to make sense. The only way a fantasy world can get away with having out-of-place biomes, is if the author creates a reason for this part of the world being out of character ('magical influences' being a big scapegoat here). If Skyrim is a cold climate, comprised of snow and tundra, then suddenly crossing a mountain range into jungle, without slowly progressing through temperate regions first, wouldn't make sense, and so would have made Tamriel less believable.
Bethesda fixed this with Oblivion, transforming northen Cyrodiil (and presumably also southern Skyrim) into Boreal forest, hence all the confierous woodland, and central Cyrodiil into temperate grassland and forest. As the world progresses further south, we find tropical and sub-tropical forests (jungles) inside Valenwood and western Elsweyr. Trust me, when Bethesda made this lore correction, it made the world of Tamriel
much more believable. ^_^