That's not unusual. It can just be the distance and number of walls between the router and your phone. Putting the router somewhere higher might help (e.g. on top of a cupboard instead of on the floor).
Could also be that the router isn't on the 'main' socket. Try moving it to the first socket after the wire comes in from outside.
Alternatively, it could be interference from your neighbours' WiFi (a particular issue in high density areas like apartment blocks). There are some free tools for the PC that let you check which types of WiFi are least crowded so you can switch your router to use those.
If you're using the router your network provided, chances are it's a cheap one - a better one could make a big difference.
