Perhaps because it’s not an error. 
Unless you were just confused… 
Hence the quotation marks…
I thought so… 
Spot on. Thanks Whiddler. I don’t believe it’s wrong at all. No offence meant or taken on my part.
Mick
Whatever works I guess… My teacher plays with left hand on top but with four fingers and with two fingers right below (hence his nickname Four Fingered Fré). That works too (though it’s sometimes hard to convert his ornaments to my “regular” style of playing).
A for me was especially a hard one to master, B was also quite difficult. Independent control of one’s weak hand fingers, especially the third/ring finger doesn’t come easy for a lot of people, especially if you’re learning this late in life.
What happened to me was that i’d sussed out the mechanics of the rolls but just couldn’t get my fingers to obey. Then one day two years ago i got laid up with a rather nasty infection for four weeks, so i simply muted a whistle right down as low as it would go and laid on my back playing rolls to a metronome ticking for about four hours a day - four the whole four weeks. Yeah, i was bored, being laid up does that to ya. 
So i’m definitely in the Bloomfield stable on this one. Get a metronome and slow it down until you get the cut and tap working on time and clear. It may be awfully slow at first but with practice you’ll soon be clicking up the metronome speed.
Of course i don’t recommend getting laid up with infections, you can instead carry a well muted whistle around with you everywhere you go and any time you get a few moments just pull it out and practice your rolls.
And i second the person who recommended The Kesh. It’s amazing how many A and B rolls you can comfortably fit into that tune. And after two years of practicing it with the gnome and slowly increasing speed i can now play it along to the Bothy Band.
It takes time and a lot of practice - so don’t give up!
Another one for A rolls is Fraher’s Jig (i think that’s what it’s called). A rolls to low D crans over and over, lots of confusing fingery fun!