Differences between silver jewelry alloys
Stainless steel ( assuming 316 ) is more corrosion resistant than silver and would not darken in air. Silver darkens with exposure to hydrogen sulfide ; yes I know there is "no" H2S around the home. However some smell from eggs is a ppb ( part per billion = not very much)  of H2S , and most of that and sewer gas H2S finds and reacts with any silver turning it dark. Most have experienced putting silver in a drawer or box and finding years later that it has darkened ( from sulfide). 92.5  % is the minimum silver content to be called sterling silver, so presumably both items are silver, the balance is primarily copper,  but may have a little nickel or zinc. My guess is that the brighter piece has a plating like rhodium to prevent the usual darkening of silver. But the real answer is that a jeweler would need to examine it.