The key feature of "scalability" that most people care about is actually the ability of a system to efficiently convert money to increased capacity
Almost any small web service could have $10,000 thrown at it and get faster. A new server. More memory! A better load balancer. But you won't see ten times the benefit if you throw $100,000 at it. What would you get? Lots more memory? You'll just bottleneck somewhere else. Ten more servers? Your database won't take the increased load. Maybe it's the salary of another developer. But that won't get you a x10 speedup either.