Why Soviet project numbers resist a simple chronology

How do the Stalingrad-related Soviet project numbers fit into chronological order?

They do not map cleanly onto chronology because numbering could follow initial conception, while later variants retained numbers despite substantial redesign.

Project numbers could reflect when an idea was first recorded rather than when detailed design or construction began. That means a numerically later project might have reached a more advanced stage before an earlier-numbered one. Extensive variants could also retain an original number after evolving enough to resemble a different project. The result is a design lineage that needs more evidence than numerical order alone.