Oh, it really is all about luck. Honestly, I always get 5, 6, and 7 seconds on Beginner, but on my old work computer I actually got 2 seconds once, and I was confused after I got it, because I didn't mark 1 mine, it was just that lucky.
The same really goes with Intermediate and Expert. I usually get in the 40's, and low 50's when I play, but every once in a while, you get one of those good boards, where there is no guesses, and all the sequences fit perfectly, making it really easy.
Expert, I've actually only broke 100 maybe 4 or 5 times, it just happened that the last time I did, I fucking demolished it! As you can see.
Oh, and in case you are wondering why my times are pretty leet, its all due to my first Co-Op (Paid internships set up through my University for 6 month periods). I worked at Lockheed Martin, which is a Naval Defenses company. Of course, I needed a security clearance. It goes Restricted clearance, which is the least access, Secret, which is a lot of access to sensitive material, then Top Secret, which is all the badasses you see on the Defense Committees on TV all the time that can't disclose anything. Well, for my job I needed a Secret Clearance. For this, they needed to do an intense background check on me. Well, this lasted for a little under two months, which is when I literally had NO work to do, as well as a group of about 10 or so of us in the same boat, so we all played Minesweeper all day and tried to beat each others record. I couldn't even beat Expert when I started, but after that two months, I was awesome. Figures.