Thank you, Mavs fan. It surprises me too, but in order to see what's going on, you have to look at the season as a whole. Even through their roughest stretch of basketball this season, the Spurs were still among the top five teams in terms of winning percentage and point differential. Their defense has improved steadily over the course of the entire season.
Some fans look at the season differently, and that's why you have some differing conclusions. I don't think a three-game stretch here or a five-game stretch there is as useful as looking at all of it, and a three-game losing streak can create a sense of panic. No team is as good as a winning streak indicates, and no team is as bad as a losing streak indicates.