Wasn't 90% of Arena retconned though? The location of Ebonheart? The settlements on Vvardenfell not being present in Morrowind? Khajiit being human, and Argonians looking like humans with scales?
Yup allot has been retconned.
Though you can't describe every change as a retcon. It's also possible to expand and elaborate on already established topic. The Khajiit would be a good example of that. Were you first only had humanoids with fur, we now have 16 different types of Khajiit, one of them being humanoid with fur.
Neither is every retcon significant or bad. Places moving about or the outline of the maps changing aren't that significant when they're just dots and lines on a map. When they have a history it's different, then a whole history has to be changed.
The Jungles of Cyrodiil however have been http://www.imperial-library.info/pge/cyrodiil.shtml and are even http://www.imperial-library.info/obbooks/provinces_tamriel.shtml in Oblivion, http://www.imperial-library.info/obbooks/before_the_ages_of_man.shtml. So referring back to Arena and considering it to a be a retcon doesn't cut it.
But the missing jungles are only the most iconic element of a much larger vision on Cyrodiil. Cyrodiil is a place that has had more then three thousand years of civilized history. In that time what used to be an elven city expanded out into the islands and marches of lake Rumare. The city has seen the rise and fall of Magocracy, religious hegemonies, trade and cults in all sorts and forms. But you can read the First PGE for yourself.
I can understand that no game could ever catch all the intricacies of this vision, but this absence isn't something that can be explained by a retcon or something willing suspension of disbelieve should be expected to cover for.