I'll try. First thing you have to remember is that there have been far more Army units throughout history than Marine, and a far greater diversity in terms of unit types.
As I understand it, it is based on the old Regimantal system started by British and then adapted later to the Brigade/Divisional system. Unit designations are dependent on the unit type, so there can be more than one 1st Regiment, or Brigade, if they are different types of units e.g. infantry, cavalry, artillery, etc.
From there, each new unit of that type is given the next numerical unit--1st, 2nd, 3d, etc. Sub units (batallions) of that unit follow the same order--So the only MP Bn, in a brigade or division will receive its numerical designation from previous MP Bn's, not its order within the parent unit. Usually, company sized units do not receive numerical designations, but some specialized units like MPs, engineers and the like also follow this rule rather than simply being designated A, B, C.
Once a unit designation is given, it can never be handed-off to another unit if the original unit has been permanently demobilized. So the progression continues ever higher.
In the case of the 101st Airborne Division, it is the 101st infantry unit of BN or larger size to enter service.
It's confusing, and I don't claim to have this 100% right, but it does make sense when you consider how it is put together. Historically, the Marines have been a much more stable organization in terms of size, type and number of units, so its number system is not as confusing.
It is also helpful to understand that a new division can be formed from the constituent parts of former divisions.
Do you think this hurts unit morale or the esprit of the troops?
Semper Fi!
TS