Esto te pasa porque son 2 condiciones, no 1, sobre las que esta el pivot.
Código SQL
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SELECT
IDMuestra,
max (case when IDLab = 1 and IDMuestra = 'Sample-01' then TenorAu end) as Lab1,
max (case when IDLab = 2 and IDMuestra = 'Sample-01' then TenorAu end) as Lab2,
max (case when IDLab = 3 and IDMuestra = 'Sample-01' then TenorAu end) as Lab3
FROM
(
SELECT 'Sample-01' IDMuestra, 4.5 TenorAu, 1 IDLab
UNION ALL
SELECT 'Sample-01', 6.5, 2
UNION ALL
SELECT 'Sample-01', 8, 3
)a
GROUP BY IDMuestra