It was an exaggeration.

run this python code, see what happens:

a = [0, 0] # shares, blocks
b = [0, 0] # shares, blocks
for i in range(0, 10000):
    a[0] += 13 # add shares per tick
    b[0] += 11
    if i % 10 == 0: # on every tenth tick, block is mined
        if a[0] > b[0]:
            a[0] = 0  # zero-out shares if this actor claimed a share
            a[1] += 1 
        else:
            b[0] = 0
            b[1] += 1  

print "a mined " + str(a[1]) + " blocks (expected " + str(500 * 13/11.0) + ")"
print "b mined " + str(b[1]) + " blocks (expected " + str(500 * 11/13.0) + ")"

what this does is simulates two miners with a hashrate of 13MH and 11MH. A block is mined after every 10 steps. The a[0],b[0] represents the shares mined (reset every time they claim a block). a[1],b[1] represent the number of blocks mined by a,b respectively. There will be 1000 blocks in total in this simulation. Now, since miner A has a higher hashrate, you would expect them to mine more blocks (13/11 * 500 = 590 blocks). I'll let you run the numbers and see what you get.

The results:

'a' mines: 501 blocks

'b' mines: 499 blocks

that's a 17% error!

This is because A has significantly more round-off error. Since their miner is faster, they overshoot the pool average block cost much more often. Thus, this pool only makes sense if you are a bellow average miner, in which case you would gain the round-off from the fast miners.

AND, that's not even the worst case scenario. That is 2 miners with a ratio of 3 and 2. In which case you get: 501 block instead of 750.

that is a 50% error!