In what is being billed as "the $25 million misunderstanding," Washington suburban legislators discovered today that what they thought was a carefully calculated strategy to obtain state funds for future Metro construction may leave them empty handed.

The misunderstanding occurred Saturday when legislators met with Gov. Harry R. Hughes to discuss the need to push legislation committing the state to spend another $25 million for the next leg of the Metro rail system.

Senate leaders from Prince George's and Montgromery counties say they decided to withdraw that $25 million bill after the Saturday session because Hughes had promised to transfer money to Metro from state transit funds by administrative action later this year.

Hughes denies that he ever made such a pledge, saying at a press conference today that he would not come up with the funds without prior leislative approval.

"I never made any such committment," he said. "Under no circumstances can I make it [ $25 million] available without legislative approval, nor would I. As usual, there are a lot of misunderstandings that go on in the halls of Annapolis."

One floor down from the news conference, two senators who met with Hughes on Saturday were exulting over their legislative coup, not realizing that at almost exactly the same moment the governor was flatly denying their statements.

As they sat in the Senate lounge they said they had achieved the top legislative goal of their counties-another year of funding for Metro construction-without having to encounter the strong opposition that was forming in the Senate against their bill.

"What we've done is a little end run," said Sen. Victor Crawford, chairman of the Montgomery delegation. "We're getting what we want after the sessions without a bloody filibuster."

Sen. Peter Bozick, chairman of the Prince George's delegation, piped in: "The governor told us we've got the $25 million. He said you don't need legislative approval. The governor's an honest man; I feel satisfied with his word.

That was Bozick and Crawford talking before they heard Hughes' side of the story. As they listened to a tape recording of the news conference, their composure changed, although they still had a hard time acknowledging what had happened.

"We'll get the money, the governor promised and he's the governor," Crawford insisted, running off without further comment.

"There's an awful lot lost betwixt the saying and the dong," a stunned Bozick added.

Was the more said at the meeting? he was asked.

"On this Bible, I don't know of anything else," said the Prince George's senator, reaching into his pocket for a green plastic cover edition of the New Testament.

The questions persisted and the two senators began realizing that they may have miscalculated-and that perhaps they had agreed to back off on their primary goal of the seesion on the basis of a misunderstanding.

"We had, I thought, a consensus, an understanding," interjected a confused Crawdord, bobbing his head out of the Senate door. "Now, I guess we don't. It's most distressing."

"I was expecting the money April 10 [the first day after the session ends]." said Bozick. "He kept saving you don't need legislation. Obviously, I'm surprised."

At issue in this peculiar legislative saga is funding for the next phase of Metro's 101-mile rial system. The state already has appropriated $160 million for the first phase, but that money is expected to run out at the end of next year.

Washington suburban legislators originally hoped to get a state commitment for the full costs of the second leg- $156 million over the next five years. They agreed to settle on an extra $25 million last week after Hughes promised to support that more limited effort through legislation.

Although they agree there is no immediate need for full funding, they say their county governments need assurances of at least $25 million this year so they can make up their budgets for fiscal year 1981.

The senators still were shaking off their surprise late today and were still can revive the bill they had just decided to withdraw or they can try to reach a new understanding with Hughes.