Mosfet shared diffusion Spice extraction (Magic)
Eventually I found out that in SPICE tools the AD AS PD PS values are calculated node by node. Like, if there are more than one transistor terminal connected to a node, the SUM of all corresponding perimeter/area values are calculated for that node.

So in my case since all the transistors are in parallel, all perimeter/area values corresponds to the sources(and drains at the other node) are summed up and written by the first transistor. The result would be the same if they were written seperately. So it is not an error or bug that Magic does.