Moo's Point
Moo brings up an interesting point. There is a world of difference between simply flipping cards and keeping whatever counting/tracking system you may want to utilize and actually playing out hands AND counting down a deck(s). We can add to this further yet and add in the bells, buzzers, noises and distractions of live casino play to that even!
Something I noticed way back when is that even though I could count down the cards with some sort of reasonable proficiency, which I was practicing long before I ever strolled into an actual casino, I found that I could go off on the count due to the noises in a casino. The bells, the buzzers, the various boistrous noises, loud people nearby yelling their fool heads off, waitresses with large breasts, etc. Distractions that can have an effect on keeping an accurate count.
Flipping cards is easy. Flipping cards and playing hands while counting is harder. Cards flipping and playing proper strategy while noises and interruptions are going on is harder than that even. An oriental gentleman screaming "Monkey Monkey" at the top of his lungs right next to you while the drunk to your left is about to throw up all over your shoes as the waitress is shoving her breasts in your face while handing you your beverage and a slot machine nearby is clanging loudly from some little old lady who "hit the big one" and is also screaming like a lunatic and you never lose count and make the proper strategy move in a cool, collect manner while all this is going on against the fastest dealer in the house----THEN YOU ARE ON TRACK!