my guess is, that team name is defined as text. also the "AND my_score ..." is pushed up against the team name - if this is correct, you're missing the surrounding aposttophes, so your query would be:
"SELECT COUNT(my_team) AS wins FROM matches WHERE my_team='" & team & "' AND my_score > opp_score"
You could have a nice overview with this query:
,count(*) as wins
group by my_team
order by wins desc