It might have something to do with the coding of the game. It's more logical and easier to use units as opposed to hexagons, which are measured in units anyway, to calculate unit range changes. But Riot might've lazied out of doing the transformation from units to hexes, when it comes to UI infos, and just took the numbers from the unit code and slapped it onto the info tabs of units. Which might be understandable since the game is still relatively new and there are a lot of fixes to be made later on, following a schedule, whatever that is.
But, I do agree that, as an information shown to the player, in the UI, where the board is measured in hexes, it should be in number of hexes. That would make it clearer and simpler to read and understandable for every type of player.