IsNumber fails cast while IsBigInt passes cast

/*
-- Tests pass isnumeric AND fail IsBigInt AND fail cast(vc as bigint)

-- range
SELECT IsNumeric('-9223372036854775809'), dbo.IsBigInt('-9223372036854775809')
SELECT IsNumeric('9223372036854775808'), dbo.IsBigInt('9223372036854775808')

-- invalid chars
SELECT IsNumeric('-5d2'), dbo.IsBigInt('-5d2')
SELECT IsNumeric('-5e2'), dbo.IsBigInt('-5e2')
SELECT IsNumeric('+3,4'), dbo.IsBigInt('+3,4')
SELECT IsNumeric('+3.4'), dbo.IsBigInt('+3.4')

-- pass this strange case
SELECT IsNumeric('00000000000000000000000000001'), dbo.IsBigInt('00000000000000000000000000001')
*/

IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'dbo.IsBigInt') AND type IN (N'FN', N'IF', N'TF', N'FS', N'FT'))
DROP FUNCTION dbo.IsBigInt
GO

CREATE FUNCTION dbo.IsBigInt (@a varchar(30))
returns bit
AS
BEGIN
	-- Submitted to SqlServerCentral by William Talada
	DECLARE
		@s varchar(30),
		@i int,
		@IsNeg bit,
		@valid int

	-- assume the best
	SET @valid = 1
	SET @IsNeg=0
	SET @s = ltrim(rtrim(@a))

	-- strip OFF negative sign
	IF len(@s) > 0
	AND LEFT(@s, 1) = '-'
	BEGIN
		SET @IsNeg=1
		SET @s = RIGHT(@s, len(@s) - 1)
	END

	-- strip OFF positive sign
	IF len(@s) > 0
	AND LEFT(@s, 1) = '+'
	BEGIN
		SET @s = RIGHT(@a, len(@a) - 1)
	END

	-- strip leading zeros
	while len(@s) > 1 and left(@s,1) = '0'
		set @s = right(@s, len(@s) - 1)

	-- 19 digits max
	IF len(@s) > 19 SET @valid = 0

	-- the rest must be numbers only
	SET @i = len(@s)

	WHILE @i >= 1
	BEGIN
		IF charindex(substring(@s, @i, 1), '0123456789') = 0 SET @valid = 0

		SET @i = @i - 1
	END

	-- check range
	IF @valid = 1
	AND len(@s) = 19
	BEGIN
		IF @isNeg = 1 AND @s > '9223372036854775808' SET @valid = 0
		IF @IsNeg = 0 AND @s > '9223372036854775807' SET @valid = 0
	END

	RETURN @valid
END
go