Strava bases estimated time on your past 4 week average speed. If you want a better estimated time, Best Bike Split is probably your answer. It would be awesome if Strava incorporated something like BBS in their route builder.
“Your estimated moving time for a route is calculated by your 4-week average speed or pace. On your profile, you’ll see your average distance per week and your average time per week based on your last 4 weeks of activities. We take that average to estimate your moving time for the route. If you share the route, the estimated moving time will be adjusted based on the viewer’s profile stats.”