Stayman in Doubt is a method to allow the Responder to the 1NT opening to state that he has a flat hand with exactly a balanced 4333 distribution, the 4 card suit matching with Openers 4 card major and enough points for game.
Although agreeing the trump suit, showing a 4:4 trump fit, with this flat distribution he is allowing Opener to play in 3NT if he too has a 4333 distribution.
Although the Partnership has a combined count of 8 cards in one major this will be of little benefit to them as the exact flatness of the hand provides little or no chance of Declarer being able to make an extra trick with a ruff.
Therefore there is probably more chance of making the 9 tricks for 3NT with a balanced hand and 25+ High Card Points than 10 tricks in the Major.
After the sequence:-
1NT - 2 ♣ - 2♥ or 2 ♠ (no interference)
3 ♦ is Stayman in Doubt
Openers options are;
3 of the major with a minimum hand; (in case responder is thinking about a slam) or,
3NT with a maximum hand and 4333 distribution; or,
4 of the major showing a maximum with a little more shape [4432, 5332].
To overcome the same problem with an invitational hand responder would agree the Major with, say, 3 ♥ and Opener will Pass with any minimum, bid 3NT with a maximum 4333 or 4 ♥ with a maximum and a doubleton somewhere.