I had targeted my ads to only US. I kept getting hits from other countries.
Later I found out that GEO TARGETING for site match is at DOMAIN level and NOT at IP level.. i.e. if you target for US, then it will appear on yahoo.com, if you target for UK, it will apprear on yahoo.co.uk. User's country does not matter for this targeting.
This has made Site Match less interesting, at least for me.