If you look again at the picture, there is the suggestion of a slight gradient there. However, it's not really enough to cross down to another deck.
The whole turbolift scene is a bit silly really. Kirk never even presses a button for his destination, yet it takes off and delivers him there anyway. And if the character on his exit door is indeed a "D" then he had progressed down just two decks at an absurdly slow rate - he could have crawled down the stairs quicker!
I realise that turbolifts often move at the speed-of-plot but this is setting a new standard.