It's amazing how hard communication can be! My simple sentence, "Finland is a NATO ally; but sure Canada makes sense too." has three interpretations - all reasonable IMHO:
1. cgh's: 'Finland is ok because they are in NATO; Canada, who in contrast are not in NATO, also makes sense.'
2. yours: 'Finland is an ally of NATO, not a member.'
3. mine: 'Canada may have 'close economic and cultural ties' (from the parent comment), but Finland is a member of the NATO alliance. Canada still makes sense (no comment on their NATO membership, which is a fact).'
I think some is context: In what I read, 'NATO ally' is a somewhat common term for members of NATO (an alliance, of course), and I instinctively used it. It's hard to remember that other people read their own materials.
Perhaps that's what you meant, but Ally makes it sound like they're friendly with NATO rather than an actual member.