Basically, the results of compression test carried out by using cubes are higher than that by cylinders.
In compression test, the failure mode is in the form of tensile splitting induced by uniaxial compression.
However, since the concrete samples tend to expand laterally under compression, the friction developed at the concrete-machine interface generates forces which apparently increase the compressive strength of concrete.
However, when the ratio of height to width of sample increases, the effect of shear on compressive strength becomes smaller.
This explains why the results of compression test by cylinders are lower than that of cubes.
Reference is made to Longman Scientific and Technical (1987).