It's hard to tell why the job went so well. I can't say what type of 'material' the wallpaper is. I don't know exactly when they were 'invented' but there are vinyl, paper, and 'cloth' type wallpapers. The most common problem with painting wallpaper is that the paint usually gets absorbed by the paper, and when it dries, it shrinks and cracks. If you can determine if the paper is a vinyl or cloth type, you may be OK to paint it w/o any problems. BUT, if it's paper-paper, then you're flirting w/trouble.
I will admit that it isn't that costly to paint. If it works out, then you did the 'right thing'. IF it doesn't, then you're simply out the time and $$$ (which again isn't that much on both accounts.)
I do wish you well. My best to ya and hope this helps.
Jay J -Moderator
PS: Keep your eye on this post to see if anyone else has something to say ...