When you think of a concrete object, you think wordlessly, and then, if you want to describe the thing you have been visualizing you probably hunt about until you find the exact words that seem to fit. When you think of something abstract, you are more inclined to use words from the start, and unless you make a conscious effort to prevent it, the existing dialect will come rushing in and do the job for you, at the expense of blurring or even changing your meaning.
Concrete words are more direct in our thoughts and we use words to describe it in the best sense we can. Wth abstract words, we lose vision on what our true purpose is and the meaning is changed of the overall sentence.