Different customs and perhaps some different interpretations, but if you believe something different (e.g. that deviates from or adds to scripture) then objectively you have a different basis and therefore a different faith.
I'd encourage you to watch some of the debates by James White[1] (yes, those are links to purchase, but if you check the podcast feed/blog they link to YouTube videos of many of the debates. What you'll clearly see in many of those debates is how the arguments of one side breaks down. I can't see how any reasonable observer could watch a handful of those debates and not be at least open to if not convinced that perhaps some of what these "different beliefs" offer isn't real Christianity, but something dressed up as such.
I do believe that there are very small number of _actual_ Christians in the US. Definitely not the 70-ish% from Pew[1]. Maybe 5-10%. Probably even less.
[1]: http://www.aomin.org/aoblog/index.php/2011/01/19/available-d... [2]: https://en.wikipedia.org/wiki/Religion_in_the_United_States
Anyhow, I don't care to argue specifics, only to point out that as everyone thinks their version is correct, and there's many versions out there, it's difficult to say what a 'real' Christian believes, assuming one is charitable enough to consider sects other than one's own as 'Christian'.
I wasn't trying to list types of Christians, merely pointing out that, to a typical American Christian, evangelicalism is synonymous with Christianity. I also missed the Lutherans, Anglicans, and a bunch of others.
Edit - should add, Orthodox consider any church 'in communion' with the rest to be the same church. So Russian, Greek, Bulgarian, Serbian, Romanian Orthodox churches, as well as the Patriarchates of Jerusalem, Antioch, Alexandria and Constantinople, as well as a bunch I'm missing and too lazy to Google, all consider each other to be the same church. Imagine the Catholic church but with a dozen or more 'Popes', all more or less equal (I say more or less because the larger churches often wield more influence, even if they don't officially have more authority).
Probably because there's more Christian sects than there are flavours of ice cream. All with different beliefs.
To me, Christianity is synonymous with Orthodox Christianity, but for the typical American 'Christian' it's probably some flavour of Protestant Evangelicalism...