Wouldn;t overanalyse LH flight number changes too much:
MAN had numbers in the 070s, then we had the split of the 1600s and 5500/5600s then the 4800s and 4900s to the current arrangements of the 900s for FRA, 2500s for MUC and 3400x for the others.
All they are probably doing is tailoring a group of flight numbers for UK to FRA, UK to MUC and UK to regional German cities