But Cruz, truly was born the son of a non-US citizen father while his mother was in Canada. Does the fact that Canada is in North America make a big difference, legally?
No, it makes no difference, and if you're asking this, then you don't understand the controversy at all. To become President, you must be a "natural born citizen" of the United States. Most legal scholars interpret a "natural born citizen" be be someone who is born a citizen - meaning someone who never had to be naturalized.
People are born citizens in two different ways. First, they can be born in the United States and subject to its jurisdiction. This is the British common law doctrine of "jus soli" that became federal law with the ratification of the 14th Amendment. The second way they can be born citizens is by some statutory scheme. One of those is passage of citizenship from parent to child, but this isn't automatic. There are conditions and limitations on it.
Obama was born in Hawaii, so he was a citizen under the 14th Amendment and never had to be naturalized. However, let's suppose the birthers were right and Obama was actually born in Kenya. He would have to gain citizenship through his parents. Of course, Obama's father wasn't a US citizen, so he couldn't get it through him. He'd have to get it through his mother, but there's a problem with that. For her to pass on her citizenship to him, she would have to have resided in the United States for at least ten years, at least five of which were after her 14th birthday. She was only 18 when he was born, which means he couldn't acquire citizenship through her. Therefore, he wouldn't have been a natural born citizen (or any kind of citizen) and would have been ineligible for the Presidency.
For Cruz, there was no controversy about where he was born or the status of his parents. He was born in Canada, so he couldn't be a natural born citizen through the 14th Amendment. Like Obama's father, Cruz's father wasn't a US citizen, so he had to gain his citizenship through his mother. However, she met the residency standard at the time of Cruz's birth that Obama's mother didn't meet at the time of Obama's birth. Accordingly, Cruz is a natural born citizen.