This is simply a theory, but I believe it might provide some further understanding. Simply put, the collocation “has been” seems as though it is the present-tense phrasal verb to denote relating to the past; on the contrary, “is” emphasises its occurrence as a present event.
For example, consider the following two examples:
It has been such a lovely day so far, the teenager mused; even as she spoke, the day-star rayed down its comforting warmth, bathing her in golden light . . .
“It’s beautiful today, isn’t it?” he asked his companion, eyes momentarily hovering over the form of a pre-teen girl, his junior by only several months. As he spoke, sunlight refracted off of her cerulean eyes, causing them to appear as though they were sparkling; the light also glanced off of her short, reddish-orange hair . . .
In the first example, the term “has been” refers to something earlier (in this case, the time period), although it may, at times, extend to the current happenings—but chiefly as a supplement.
However, the subsequent example indicates the present occurrences, in present tense.