A child only automatically becomes a US citizen (unless specifically renounced citizenship - i.e. 'I don't want it') if they are born on US soil, or are born to TWO US citizens (of which one has lived in the US prior to their child's birth). If born abroad, their birth has to be registered with the US consulate or embassy of the country of birth.
And yes, there is a very big difference between two US parents, and one US parent, although after 1986 that was relaxed. I presume that you were born before 1986, at which time the law was changed to state that if one of your parents was a US citizen, they lived in the US for at least 5 years before your birth, and at least 2 of those years were AFTER your US parent's 14th birthday, you would be accepted as a US citizen.
Before 1986, the law was even more strict in the case where only one parent was a US citizen (a minimum of 10 years of residence/permanent presence in the US, of which a minimum of 5 years was after their 14th birthday).
As for your question why there can't just be one system... let's see... IRIS is UK, Clear is a similar system to IRIS in the US. Passport card is not valid on flights, only on land/sea entry into the US (so you don't have to carry a passport, but rather just a card to slip into your wallet). Nexus is a Canada/US hybrid of a passport card and Clear/IRIS, and finally Global Entry is similar to Nexus and Clear, but for international travellers.
The only UNIVERSAL system that is accepted everywhere is... surprise... your passport. It works everywhere and anywhere. Clear/IRIS/NEXUS/Global Entry are just expediters... they let you bypass the human inspection that comes with a passport (you still have to use your passport, but it's a machine and is 'faster'). The passport card is, well, a US-only invention (although, to enter the UK at Dover/Calais you can use your UK driver's license).
I for one would prefer NOT to have to share my data with the rest of the world unless I specifically say so. Clear/NEXUS/Global Entry are all for entering the US... who/where is the barrier here? I think we know that answer. The UK's IRIS system is supposed to speed things up, but I don't know... I've seen people get very frustrated and exit the IRIS queue, only to join ours for the passport. A botched attempt to build a Clear-like system?
S.