a0217. Visa numbers were not allocated upon receipting, and have not been for decades since quota were established. Several thousand were returned from missions, that's one reason, along with many other file processing anomalies inherit with large numbers of files processing and multi-part processing. To illustrate two of many factors: 1) they are not processing applications in literal direct sequence, nor 2) reporting accurately on waiting times on USCIS. These waiting times are actually not all that relevant other than a very broad guide.
Files are pulled from a document archiving service via a batch number, and within that batch are a range of unsorted applications, although the broad range of receipt dates is consistent with stated criteria for selection. Once these files are pulled, they are allocated to reviewers for a check from a list to ensure what (allegedly) went in to storage comes out, along with a review of any expired attributes of the process, then onforwarded to an approval officer if all is still current. The actual final approval and allocation of a visa number is asynschronous to the actual receipt date as there are many concurrent processes during this period. This is why, despite an effort to process as close to FIFO as they can, there are broad fluctuations in that process. This process has changed somewhat since June to promote partial approval - such as AP but more particularly EAD, but the steps are still broadly the same.
There are other influences as well associated with work flow management, but I won't go into it here.