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I agree what you are saying that Arsenal problem is not that they are not consistent but that another team is even more consistent. You are right that Arsenal got a lot of points in last two seasons 84 and 89 which is very good but not enough to win championship. This shows that Manchester City has set very high standard so other teams have to play extremely well to compete with them. Now.it looks like Liverpool is also trying to get a lot of points almost 90. This kind of strong competition is what makes Premier League so exciting.
You just confused me with the comment you just wrote. The only way to win the title is for Arsenal to keep their performance consistent or more simply, they have to get the most points at the end of the competition. Arsenal failure was because they dropped too many points, meaning they were unable to maintain consistent performance. This season Arsenal have had the same problem again, when they managed to give Liverpool fierce competition in the title race, they actually lost points in games they should have won.