The extremely high increases in the first period may be due to network entries with unknown start dates.  While the growth rates on either side of 1991 are accurate, many of the networks listed as being assigned in July 1991 may represent when MILNET was folded into the ARPAnet.  In both cases, the actual dates these addresses were first assigned are probably distributed among the preceding months.  The trend lines in the following graphs are based on the growth of the assigned address space afterwards to avoid skewing the results.