I think it's a comparison to soccer football, where one flukey goal could decide a match that ends 1-0, after the worse team puts 10 men in defence behind the ball.

A fluke can happen between two similar teams but between two very different ranks, it's difficult to fluke.

Rugby has little provision for flukey, consistently huge territory gains since forward passes are illegal - in general, the ball needs to be carried by a human being to make territory gains (kicks are available as an option but there are rules such as the man catching the ball has to be faster than the ball wince he needs to start running from behind where the ball is kicked).

You need to have a team that has some combination of being far more nimble, more powerful to explode through tackles, well-oiled together to create and change formations on the fly etc.

I don't know if it's a professional thing too but in my experience at high school level, the best defence in rugby is keeping the ball in your opponent's half - which means constantly attacking and maintaining the territory gained. That means even if a team scores a full try (worth 7 points, maximum possible single score), they need to keep attacking to defend their lead, unlike soccer football where you can stick 10 men behind the ball with 6 in the penalty box.

You can also have cheese where the kicker gets all the points from penalties and drop goals if the team is built for that purpose in mind. These points are worth less (around half) than carrying the ball over the line but they can accumulate pretty quickly if you have a good kicker.

Basically over the course of 80 minutes and constant forced engagements, the better team will generally win. If you have two teams that are very far apart in skill, a flukey win is highly unlikely.