In Magic the Gathering theory, one of the fundamental articles is "Who Is The Beatdown" by Mike Flores. Without getting too deep into Magic theory, the idea is that in any match up of two decks, one of them will have to take an aggressive role to win, the other a controlling role. Even in a match up of two highly aggressive decks, one of them will be more aggressive than the other. If the less aggressive one tries to play aggressively, chances are it will lose. If you are playing a mid-tempo deck, when you play against a fast beatdown deck, you have to play a controlling playstyle, because you can't beat the beatdown deck in a race. When the same mid-tempo deck plays against a true control deck, it has to switch to a more aggressive style since the longer the game goes the more likely the control deck will have the chance to assert control and win.
So how can we apply this to Warhammer (both 40k and Fantasy)? By asking the question, "who is the gun line?"
At a tournament, when you and your opponent exchange lists, the first question you need to ask is, "can I outshoot him?" If the answer is "no" you are not the gunline. You next question is, "can my army beat his in close combat?" If the answer is "no" you are not the beatdown. The last question is, "can I out maneuver him?" Hopefully, the answer to one of these questions is a "yes" because otherwise you have a tough game on your hands. But most times, a good all-comers list will have a yes answer to at least one of these questions. Getting that right, as we will see, is the first step in avoiding losing to a "bad matchup."
Often times players will get a match up, lose, and then tell their buddies, "I couldn't help it, I got a bad match up for my list." Sometimes there is nothing you can do and his list was truly a bad match up for you. Other times, you could have pulled out a win, except that you didn't change your playstyle to suit the circumstances of that match up. If you are a Razorwolf player, most times you will be the gunline. But in the few match ups where you aren't, you better be able to identify that, and you better be able to change your tactics to accommodate the circumstances of the matchup. In theory, a good all comers list should be able to adapt to situations where your square peg is presented with a round hold. If you don't realize the hole is round, though, that's not a bad match up, it's a bad player.
Let's say you are playing a Razorwolf list. In round one of the tournament you are matched up against a mech guard list. You will not be able to outshoot the mech guard. You are not the gunline. If you try to sit back and outshoot them, you will lose. You will have to win by maneuver and close combat superiority.
Round 2 you are matched up against 2 Battlewagon Nobs Ork force. You can outshoot him, and you cannot beat him in close combat. You are the gunline. You have to try to win by superior fire power because you cannot move him in close combat. On the other side of the coin, he has to play aggressively and try to assault you because he has to realize he can't outshoot you. If either of you confuse your roles and try to play the wrong style, you will probably lose.
Round 3 you are matched up against another Razorwolf list. This is where it gets interesting. In your list you have an extra squad of Long Fangs, and he has an extra squad of Thunderwolf Cavalry. It is important that you realize that you are the gunline, and he is the beat down. If you push forward and try to out-assault him, you will probably lose. If you utilize your superior firepower your chances of winning are probably much greater.
As Mike Flores says, "Miss-assignment of roles = Game loss." In 40k and Fantasy, we have to remember that the chances of having a true tournament mirror match where we play against identical lists are relatively small, but in GW systems there are 3 ways to win: shooting, combat, maneuver. When you are matched up against an opponent you have to know which one you are in that match up. In 90% of matchups your Razorwolf list is the gunline, but being able to identify- and then accommodate to- that 10% of exceptions is an important skill that will give you a greater chance of winning "bad match ups."