Miner A will only find a share in time approx 50% of the time. So half the time he has 1 share, half the time he has 0 shares. He will have worked an average of 15 seconds for nothing when the block changes.
Miner B will find an average of 10 shares. He, as well, would have been working towards another share when the block changes. However, on average, he will have only been doing so for 1.5 seconds.
So the difference for Miner A is the difference between all or nothing.
The difference for Miner B is between, say, 9 and 10 shares... or maybe 10 and 11 shares...
Miner A can get 0, 1, 2, ... etc. He could be lucky and get one block every 5 seconds, for example.
The actual odds are
0: 0.3673
1: 0.3685
2: 0.1842
3: 0.0612
4: 0.0152
5: 0.0030
6: 0.0005
7: 0.0001
8: 0.0000
If you work it out you get an average of 1 block.
For a miner with 10X the power, the distribution is
0: 0.0000
1: 0.0004
2: 0.0020
3: 0.0070
4: 0.0179
5: 0.0366
6: 0.0620
7: 0.0897
8: 0.1133
9: 0.1268
10: 0.1272
11: 0.1157
12: 0.0961
13: 0.0734
14: 0.0519
15: 0.0341
16: 0.0210
17: 0.0121
18: 0.0065
19: 0.0033
20: 0.0016
It is balanced around 10.
Again, if you work it out, it averages to 1.
However, the more powerful miner gets lower variance.