When a sorted set's members have the same score, they are sorted lexicographically. There's no straightforward way to get them ordered differently AFAIK but you can easily do the sorting on the client's side. Alternatively, you could cook up a short Lua script that does that for you on the server.
There is a slightly hackish way that you can get away with what you want without resorting to the approaches above. Based on the example you had provided and ssuming that both rank and user id are integers, you could use scores that are a combination of both, like so:
zadd game_rank 550006435 6435
zadd game_rank 556088561 6088561
zadd game_rank 550608825 608825
This will let you do ranges and you'll get "numerical" sorting for each rank (i.e. with zrangebyscore game_rank 550000000 559999999
).