$18 on Frictional Games Store and only $16 on STEAM

[Whyarentnumbersvalidinyourname]

I'm not asking for the $2 back I just wanted to know why as it seemed at little odd for them to offer 20% off on Steam and only 10% off from their store.

Now what would annoy me is if Steam pre-orders were able to pre-load and Frictional Games store pre-orders had to wait until release for keys.