How to interpret Strava power curves?
You were probably doing intervals, or at least your effort was at a high effort for several minutes, followed by a recovery for several minutes, and another high effort rather than an extended "steady" output to exhaustion. 

In general, you're right, the maximum mean power for a duration should be a monotonically decreasing function of duration. However, for any single ride, especially when you're doing intervals, the MMP for that ride needn't be monotonic. 

Here's a simple (too simple) numerical example. Suppose you were doing repeated 5 minute intervals, with 5 minutes at 325 watts followed by a 5 minute recovery at half that, or 162 watts. If you were to calculate the MMP for a duration of 9:00, it would be 252 watts; the MMP for 13 minutes would be higher than that, at 262 watts. You didn't do an extended effort for either 9 or 13 minutes but that doesn't prevent the arithmetic of calculating an average power over 9 or 13 minutes. In your case, it appears that your workout was on an ergometer and involved at least one 1 minute interval at 450 watts. Depending on how long your work and recovery intervals were, and at what levels, you can see non-monotonic MMP curves.