1. Their opponents have less time to adjust to them: If you are only playing from time to time on a few tables, your opponents will have a hard time to get stats and reads on you. Furthermore since they rarely see you at the tables they won't feel the need of deeply analyzing your game to find weaknesses/tells.
Additionally you can get away with playing out of line often times (for example raising lots of rivers). Your opponents will not notice it. But if they play against you on a daily basis, they will recognize if you are out of line and adjust accordingly.
After playing lots of hands against the same opponent you will have a certain history with/against this villain. This history can lead to complicated decisions and wired game dynamics. When you are playing in a small player pool, you will have history with most opponents pretty quickly.
2. They are able to table select better: If you are only playing the very best tables available, your chances of a better winrate are highly increased for obvious reasons.
3. On average, they have more time to think about a single decision.
4. They have a smaller player pool to adjust to: People who are playing lots of tables at the same time have to play on different stakes and table sizes (FR, 6m, HU) at the same time in order to find enough tables. This means that they face lots of different opponents. Facing a larger pool of opponents leads to far more complex situations that are avoidable by playing less tables.
Important Note: Like I already said in my previous post ("Play Good or Play a Lot?"), increasing our winrate doesn't necesserily lead to increased winnings. And winning as much as possible is the ultimate goal.
Conclusion: We shouldn't overestimate our winrate. It is highly influenced by variance, our game selection, our game time selection and our volume. The more important column is the winnings column. Nevertheless we shouldn't underestimate our winrate. Over larger sample sizes it can be a good indicator for the current state of our game.
PokerStars No-Limit Hold'em, €1.00 BB (9 handed) - PokerStars Converter Tool from http://flopturnriver.com/
saw flop | saw showdown
Hero (MP3) (€108.85)
Preflop: Hero is MP3 with 10, A
1 fold, UTG+1 calls €1, 2 folds, Hero raises to €5, 4 folds, UTG+1 calls €4
Flop: (€11.50) 9, K, J (2 players)
UTG+1 checks, Hero bets €7, UTG+1 calls €7
Turn: (€25.50) Q (2 players)
UTG+1 checks, Hero bets €17, UTG+1 calls €17
River: (€59.50) K (2 players)
UTG+1 bets €24, Hero raises to €79.85 (All-In), UTG+1 calls €55.85
Total pot: €219.20 | Rake: €3
UTG+1 didn't show
Hero had 10, A (royal flush).
Outcome: Hero won €216.20
When the traffic is stuck, I sometimes add 100nl to my game selection as I did in this hand back in 2011. Villain mucked KhTh.