So if you create one backup per night, for example with a cronjob, then this retention policy gives you 512 days of retention. This is useful but this can require to much disk space, that is why we have included a non-linear distribution policy. In short, we keep only the oldest backup in the range 257-512, and also in the range 129-256, and so on. This exponential distribution in time of the backups retains more backups in the short term and less in the long term; it keeps only 10 or 11 backups but spans a retention of 257-512 days.

the exponential distribution is the probability distribution of the time between events in a Poisson point process, i.e., a process in which events occur continuously and independently at a constant average rate.