Ajax were the favourite against Feyenoord today but we could have every kind of result in this match. Ajax still were able to defeat them by 2-0 and gained an important advantage against them in the standings. They had a chance to take the lead back and they were the leader temporarily. But PSV didn't make a mistake and crushed Waalwijk by 4-1 and maintained their lead.
You say we could have had any result, but that is not quite true given how strong Ajax currently plays. Yes they did lose a game recently in the league, but they were still the dominant team for almost the whole match. They have just not been fully concentrated for a couple of moments and conceded the goals. Usually they are focussed enough to take those games home.