As an extreme example, take 1994, the 2 teams fighting for the championships were Williams and Benetton. Now lets say at the start of the 94 season , the FIA anounced a rule change. They announce that everyone must keep their chassis and are only allowed to change a ver limited amount of parts. The FIA then announce a regulation change in that no one is allowed to use a high nose for what ever reason. So now Benetton have just handed a huge blow as they can't change much of their car, yet their car was designed around the high nose. In to 1995 Williams change very little and win by a mile. Benetton tried what they could but the regs just screwed them over and they failed miserably.
I wouldn't call that a Williams win on merit, they still did a better job with what they had like RedBull are doing now, but what they had was better than thier opponents through luck of the regulation change.