Some of the newer Leica R-lenses that feature internal focusing, do focus beyond the infinity range. The Leica R-Elmarit 180mm 2.8 APO is an ex&le.
The reason for this is to compensate for differences in thermal expansion and contraction, that will affect some of the internal components of the lens that are involved with focusing. Because the tolerances of Leica lenses are very precise and exacting, being able to focus past infinity is a design feature that helps to deal with these situations where the lens may be used in temperature extreme climates.
When focusing, most people rack the lens in and out before stopping at the point they believe to be in focus. By having some extension beyond infinity (rather than a hard stop) it allows you to do this for any situation and is more "ergonomic" for the photographer.
>Do anyone know why some lenses can be focused beyond infinity? Leica operational instructions just says that it helps focusing. I cant understand how. =================================================================
It is to allow for thermal expansion. It is much more noticable with catadioptric ( reflex mirror ) lenses.