Contact Match rule set to first+last+city rejects imports that only match on first+last

I have Contact Match rule set to first+last+city

I try importing a file which includes

• John Hurley, city=richmond
• john read, city=martinborough
• malcolm campbell, city = whakatane

they dont' import due to being duplicates. i check my database, and find I have

• malcolm campbell, city = christchurch
• john hurley, city = christhchurch etc