AS A NEW WEEK in the ineffable Congress/executive/Iran/contra saga begins, we have a question: why has it evidently been so difficult for the folks over at the White House to speak the truth on any aspect of this matter, including even relatively trivial ones?

The president is said to be dejected by the various polls showing over the months that a large proportion of the public does not believe he is telling the truth. But consider what has been offered the public. It is hard to remember or to believe, in the wake of all the contrary information we have received, that at the outset Mr. Reagan said there was no truth to the stories that this country had been providing arms to the ayatollah, then that he hadn't known what was going on, that all the arms would have fit in a single plane, and so forth.

His aides, in an incredible act of folly and arrogance, prepared phony briefings for him so that he went before the people in both a speech and a press conference and said things that were demonstrably untrue. Mr. Reagan, tangled up in all this general mendacity, then took a new tack as the contra connection developed. It was that the key repository of information on the whole operation was Lt. Col. Oliver North, that he -- the president -- was burningly eager to get the full story from Col. North, but that he respected the colonel's rights in the face of a potential criminal charge and so implored Congress to grant Col. North immediate immunity.

Well. That was then. At the end of Col. North's first day of long-awaited testimony, the White House made a point of announcing that the president, who couldn't tell us often enough how much he wanted to hear Oliver North's story, had been otherwise occupied and too busy to "tune in" on it. As Oliver North's testimony came to grip the public's attention, the White House changed its story. Guess what? The president had been tuned in, was interested. How then to account for the two versions of the truth? An administration official explained to a New York Times reporter: "I think everybody wanted to send a message that just because a key witness was on the stand, the White House had not come to a standstill. But we made too much of an adjustment."

An "adjustment"? What is an "adjustment"? Is that what people used to call a lie? Someone said on television yesterday that this was the gang that can't shoot straight. Is it also the gang that can't tell it straight? Why on earth aren't they more sensitive by now to the fact that the president's credibility has already taken an awful drubbing? They should cut it out while all the "adjustments" that have been told since last November would still fit in one plane.