May not be random pick, but the procedure itself seems to make it random.
As an example:
Lets say there are 10 files August of which some has I-140 for AOS and some CP. If first two are AOS, they will be assigned. If 3rd to 7th are CP, these will be put aside in other shelf. Hence 3rd will be in the bottom. Now 8ths I-140 is AOS.It will be assigned. Again 9th and 10th will be for CP. So on the other shelf, 3 will be the last and 10 will be at the top. Meanwhile we see the approvals of 1,2 and 8. As they finished 10 files of August, they move to September. May be this is the way they follow. Hence we may see Aug and also Sep approvals.
One person\'s job may be putting AOS in a different shelf and CP in a different one. Another person\'s job may be to request the I-140s from Consulates. After recieving, one more person\'s job may be to put the respective I-140s from mail room to respective files which are pending. On one fine day, this pending bunch will be assigned as it has all the documents.
I guess that\'s why we see, lot of approvals of 2000Pds and at-a-time approvals of 98,99PDs now. Most of the 2000 PDs would have filed for AOS immediately after the approval of I-140. Added to this, there are lot of applications filed. So that\'s why this delay.
Appreciate comments on this. Thank you.