What is the grouping I seen in the Graph tab on Dask's Dashboard

Question

For each DTO my Flask Web Server accepts, I create a new Dask Graph and run the Graph on the DTO. The runtime of the graph is 8 seconds. A single graph looks like this:

When I stream DTOs to the web server, 1 each second, my graphs look like this:

is looks like there is some kind of grouping to the graphs. What is it?