The A3100 was originally allocated to the Guildford Bypass; the initial section between the A3 at Boxgrove and the A321 at Woodbridge opened around 1932, using that number. It is shown as open on the 1932 Ten Mile Map (right).
When the remainder of the bypass was completed in 1934, the road was renumbered as the A3, which effectively swapped numbers with it (the old A3 through Guildford and Godalming became the A3100, which it remains). This may explain why the A3100 was allocated "out of zone", as its original use wasn't!
Since then, a further A3 bypass has been created, so the road is now the western section of the A25.
A transcript from MOT documentation (National Archives file MT39/246), showing the planned change of route