How rank system work?
The details are here: https://wiki.guildwars2.com/wiki/PvP_Matchmaking_Algorithm
The short version is: your MMR is an algorithmic guess about your skill level, compared to the skill level of all other active pvp players. (eg: it’s not an absolute value for skill, just a relative one.)
It takes around 90 games to home in on an accurate assessment, assuming nothing changes in your skill in that time. So, after 10 placement matches you are about 12 percent of the way to having an “accurate” MMR.
Keep playing, and you will rise or fall until you hit the correct level. Ditto your friend, who will also do the same thing.
The initial 10 games are labelled “placement” but … that’s the only difference. They don’t actually do anything different in terms of MMR or whatever.
Win ratio doesn’t matter. What matters is who you win or lose against.
The goal of the matchmaking system is to put you with and against players of similar skill. As such, you’ll win about half your games. Other people with higher ratings and similar win ratios are playing people at their skill level.