It's OK to run a discount for Steam Keys on different stores at different times as long as you plan to give a comparable offer to Steam customers within a reasonable amount of time.

https://partner.steamgames.com/doc/features/keys

Key word being comparable as opposed to same. And its not even theoretical. Just looking at games from isthereanydeals which provides historical lows over the years from numerous storefronts shows that many games have had sales cheaper than Steam.

And personal experience too buying many steam keys over buying from Steam because the prices were lower.

You could try to argue how comparable means same, but I'd say how real world sales has steam keys lower than the steam store is what matters more, since its actually what we pay.