1873 NF 5cent And 1918 Canada

im not familiar with newfie material, NF2?

What makes the coin an obverse: 2 rather than an obverse 1 or 3 or 4? How do you tell? Different obverses and reverses are considered variety's right? Now why don't I ever see this with American coins?

There are various different markers that determine obverse and reverse numbers which vary by coin and series. Yes, they are considered varieties and a matter of fact, they do exist with US coins. Just is the case of US coins we categorize the different die pairings with VAM numbers (Morgan and Peace dollars) or Fortin Numbers ( Seated dimes) etc. Since american coins were struck in much larger quanities than Canadian coins and for many more years, there are a lot of varieties such as VAM's.

Varieties are mainly a result of there being differences (often very slight) between different dies used. Dies are used to strike the coins themselves, however, they don't last forever, as striking metal wears them down. When they are replaced with new dies, those dies aren't always exactly the same and thus varieties result such as a large 9 and small 9 or obverse 5 and obverse 6.

There are many articles available regarding this, and I'm sure many of them go into detail regarding coins and varieties. This is the one I could find using a quick google search:

http://en.wikipedia.org/wiki/Coining_(mint)