The reason you see lots of oddballs is because Colt didn't add all of the features at the same time. They used left-over parts from M16 contracts to make the AR15s, and would use up all of the old parts before transitioning to the new ones. For example, the first ARs, the SP1s, used the "slab-side" uppers and lowers for decades after the M16A1 was adopted in the mid-60s, because they had lots of original M16 raw forgings laying around that they couldn't make M16s out of. So, those raw forgings got used up to make civilian AR15s.
Featurewise, it was even worse during the "AR15 A2" transition, where many guns still had slabside lowers, skinny "A1" barrels, and so on. Again, all of the old leftover parts were used up before the new, updated parts were used. Of course, this makes sense economically, but it means that it can be very difficult to tell what configuration a particular gun "should" be in based on the model number.
Later Colt guns held a lot truer to their model number designations, because there weren't as many unusable-for-military-contract parts laying around in the inventory.