Here are my past 20 attacks and past 20 defenses from most recent to last(I went into my game log and jotted them down with a pen)...
Attacks: Japanese, Chinese, Greek, Japanese, Korean, Chinese, Greek, Greek, Japanese, French, French, Korean, Korean, French, Roman, Greek, British, French, French, French.
So I was matched on the attack with: Koreans (3x), Greeks (4x), Japanese (3x), Chinese (2x), British (1x), French (6x), and Romans 1x.
Defense: Korean, Korean, Greek, Korean, Japanese, Chinese, Greeks, Korean, Chinese, Chinese, British, Chinese, French, Romans, Chinese, Romans, Chinese, Romans, Koreans, French.
So I was matched on defense with: Koreans (5x), Greeks (2x), Japanese (1x), Chinese (6x), British (1x), French (2x) and Romans (3x)
Total number of matches combining all 20 attacks and defenses is: Koreans (8x), Greeks (6x), Japanese (4x), Chinese (8x), British (2x), French (8x), Romans (4x)
Now, keep in mind that I choose opponents based on how much loot they have and how vulnerable their bases are- not on nation (I hardly even notice that when I attack), so this is pretty random, although a few of my defenses represent revenge attacks from guys I raided. The matching system does seem to be a bit skewed towards asian and mediterranean nations and France, based on this sample- though I did get matched against british forces twice. I notice I never went up against any Germans like myself.
Coincidence? The way the matching system is designed? I have no idea. It may just be that we're hitting those nations more often because they're more popular to play. It's hard to say for sure, but I do seem to be pulling a very similar sample of nations as you- both on attack and defense.