I've set up a form with defaults that are required as input for getting a document number through a stored procedure.

And I set up the lookup rules as follows:

Here's the stored procedure:

USE [dbTest]
GO
/****** Object:  StoredProcedure [dbo].[gNDNBY]
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

ALTER PROCEDURE [dbo].[gNDNBY]
	-- Add the parameters for the stored procedure here
	@i_docIdId int = NULL,
	@i_docId varchar(4),
	@i_docRev varchar(40)
AS
BEGIN

	-- SET NOCOUNT ON added to prevent extra result sets from
	-- interfering with SELECT statements.
	SET NOCOUNT ON;

	Declare @systemYear as int;
	SET @systemYear = CAST(YEAR(GETDATE()) AS int);

	Declare @currentYear as int;
	SELECT @currentYear = currentYear FROM dbo.gNDNBY WHERE docIdId = @i_docIdId;

	IF (@systemYear=@currentYear)
		BEGIN
			BEGIN TRANSACTION;
			UPDATE dbo.gNDNBY WITH (TABLOCK,UPDLOCK)
			SET docNumber = docNumber + 1
			WHERE docIdId = @i_docIdId;

			SELECT dNBYC
			FROM dbo.gNDNBY
			WHERE docId = @i_docId;

			COMMIT TRANSACTION;
		END
	ELSE
		BEGIN
			BEGIN TRANSACTION;
					UPDATE dbo.gNDNBY WITH (TABLOCK,UPDLOCK)
					SET currentYear = @systemYear, docNumber=0
					WHERE docIdId = @i_docIdId;
			COMMIT TRANSACTION;

			BEGIN TRANSACTION;
			UPDATE dbo.gNDNBY WITH (TABLOCK,UPDLOCK)
			SET docNumber = docNumber + 1
			WHERE docIdId = @i_docIdId;

			SELECT dNBYC
			FROM dbo.gNDNBY
			WHERE docId = @i_docId;

			COMMIT TRANSACTION;
		END
END

It gives me the correct number, but skips to every third number. Any ideas why this could be happening?