1) is this site fresh (less than 1 month old), or aged site with lots of authority? These are the only 2 scenarios in which I'm thinking that you can rank #1 right off the bat, if your site is neither fresh, or authoritative it's weird and seems like you're the exception to the rule
2) did you use a professional rank tracker to check the rankings, or did you just type the keyword into your browser? Because if the latter, it's possible that google have served you personalized results (because you're logged into your google account)