Need more information. This could be caused by many factors. Such as your car development vs the other team, driver stats, push levels, fuel strategies, track evolution, etc...
Also how many laps had each compound completed when the comparison was made?
You're in a no refuelling league. Unlike a refuelling league where softer tyres are usually fitted for a shorter stint and therefore have both a weight and grip advantage, in no refuelling the only advantage SS have is their grip in the early phase of the stint. They will wear and begin to lose pace much more quickly than harder compounds so lap times converge.