There were 25 Class 143 built and they were originally numbered 143 001 to 143 025 and were renumbered 143 601 to143 625 in the early 1990s. The DMS numbers have not changed, they start at 55642 (143 601) and run sequentially to 55666 (143 625), DMSL numbers have also not changed, they start at 55667 (143 601 and run sequentially to 55691 (143 625). It can be seen that the DMS and DMSL have separate number blocks but that they differ by 25 i.e. the number of sets built.
Therefore to answer your 2 questions;
1. Are the numbers correct - YES
2. Shouldn't the numbers of each type of car go up in twos? - NO.
John