Monday, 16 December 2013
Minded to find out I discover that the reason lies in initial construction of the motorway in the 1960s. At that time, the M4 construction started from London and was built out towards the west. The original plan was to construct it around the southern side of Maidenhead and then curve it to the west of Maidenhead with the intention of sweeping north of Reading. Maidenhead itself was to be junction 8 and the junction with the A4, west of Maidenhead was to be 9 (shown below).
So they changed the plans, and in 1971 sent the motorway south of Reading instead. This though meant that the final stretch of the M4 that had been constructed in the 1960s to the west of Maidenhead was now redundant, so it was renumbered as the A404(M). Junction 9 then became superfluous due to the re-route – J10 was already allocated to Reading so to keep number consistency and as not to confuse drivers the original J8 was renumbered as J8/9.
All in all an idiosyncratic quirk - if that's not a tautology.