I recently saw 2 graphs on a flight path on BBC, shown below. For those interested, it's the path of the recently crashed Boeing 737 max flight ET302, but that's irrelevant to my question.
My question is about the relationship between the two graphs. I would expect the bottom graph to be the first derivative of the top one - with altitude increasing when vertical speed is positive and decreasing when it is negative. However, this does not entirely hold. At around 5:38:45 for example, the altitude clearly drops, while the vertical speed remains non-negative.
Am I missing something about the definitions? Like, vertical speed being defined relative to the wind and the plane stuck in a down-gust. Or, if or example, is there some well-known error caused by the way these quantities are measured by the instruments?