The A4380 number is anomalous, as it lies entirely east of the A5 and would therefore have an A5xxx number if the usual zoning principles were observed. And as a Central London four-digit A-road, the second digit would normally be a "2". A5211 is the lowest available, but A5241 might have been more appropriate.
(The A41 is correctly numbered because part of it, between Elstree in north London and Cosford in Shropshire (most of it, in fact) lies west of the A5)