Your bike and its components' condition are estimated through best-practice knowledge and by figuring in the type of bike you have and the amount of riding you have logged up.
With an active Strava synchronization, you automatically share the type of bike you ride on, and the distance you cover on it. This information is enough for us to estimate the condition of your components.
The type of bike has an influence on the level of stress and wear your bike and its components will be put through. E.g., a mountain bike usually rides in harsher conditions compared to a road bike, thus a mountain bike needs service more often.