Generation number is not always the maximum of all generation numbers and so generation number is not always monotonic. Steps to reproduce: Lets start with on generation N. 1] install some package (you'll have N and N+1) 2] install some other package (you'll have N, N+1 and N+2) 3] delete generation N+1 (you'll have N and N+2) 4] switch to generation N 5] install some package - you'll get generation N+1 again (you'll have N, N+1 and N+2 again)