From: Near Portland, OR
Leaves room for expansion in between!
Think interstate numbering system. Used to be they numbered the exits 1, 2, 3, 4... but they figured out that when you have to add something BETWEEN the numbers, it causes a problem. So now they number them by mile marker instead.
Just guessing on this, but seems logical.
The Interstate system also numbers the highways from west to east for N-S roads and from south to north for E-W roads. N-S roads are also odd numbers and E-W roads are even. I-5 is the main N-S interstate in the west coast. I believe it's I-95 on the east coast.
Main Interstates are either one or two digits. Spurs off of a main interstate (usually in metro areas) have the last two digits the same as the main highway, but with a third digit added at the beginning.
As for unit numbering. I have suspected, but was never sure that spreading out the numbers was to obscure things when more units were raised in wartime. If the unit numbers were consecutive, the enemy might be able to guess how many units you have raised.
WitP AE - Test team lead, programmer