Why non polar liquids wet non polar surfaces better than polar surfaces?

In semiconductor industry adhesion promoters (such as [HMDS](https://en.wikipedia.org/wiki/Bis(trimethylsilyl)amine)) are applied on various surfaces to improve adhesion between the surface and the photoresist:



> 
> On SiO2 and many metals surfaces, the air humidity forms polaric
>  OH-bonds. Such substrates are hydrophilic, but reveal a bad wetting
>  and adhesion of the unpolaric resist. Adhesion promoters make
>  hydrophilic surfaces hydrophobic thus improving the resist adhesion.
>  [...] **The methyl groups of the HMDS fragment hereby form a 
>  hydrophobic surface thus improving resist wetting and adhesion**.
>  ([source](http://www.microchemicals.com/technical_information/substrate_cleaning_adhesion_photoresist.pdf))
> 
> 
> 


But why should non polar (hydrophobic) substances wet non polar substrates better than polar substrates? (in the opposite case of **polar substances** it is clear why)


