Little will be found in taking math to the trending here....in order to understand it we have to get off-paper.
Some understanding of the racial disparity may indeed be found in the overall gender disparity to marijuana possession arrests.
These strange trends in the Philly marijuana arrests deserve a more real-world look at how these laws are enforced. It is practice not demographics where the answer might be...in my opinion.
There's something you're overlooking, in which an anthropic coincidence type of argument is powerful in terms of an explanation. It is indeed part math, but you are correct that when you say that in order to understand it you need to get off paper. The other parts are equal parts social and political.
Consider, when politicians such as Street and Nutter have "cracked down on vice and crime" around election time they tend to do so in heavily black neighborhoods
I would ask you to remember that events such as "operation sunshine" cracking down on drugs and vice, generally were effected in black neighborhoods surrounding kensington, down towards Darion street on allegheny or in North Philadelphia. Operation "safe streets" irrc was effected in West Philadelphia. They are not set up in areas such as Hungtingdon Valley, West Chester or in Amish neighborhoods down in Lancaster.
That simply accelerates a disparity that arises because of a difference in arrest rates themselves in black vs white neighborhoods across the state due to the base crime rate.
The social factors I mentioned, are ones I know first hand having lived and partied in both black and white areas.
When I was living in pure white areas of montgomery county for example, the kids smoking weed would tend to do so out of the eye of the public. In a friend's house or apartment, in the garage or way back in the woods and usually in small social groups.
That's a stark contrast to the social dynamic I've witnessed living in areas with a heavy black concentration. There are invariably large groups of black kids that tend to congregate, for some reason usually outside a 7-11 or a chinese take-out smoking weed. When the group gets large enough and loud enough, the police show up and the group scatters, only to begin to reform once again after the police leave, having arrested one or two individuals.
When you mentioned 'disorderly conduct', I immediately thought of events like that.