Basically, it's weighted. Like you mentioned, a 22 man Triarii maniple with 3 gold chevrons will drop significantly in experience when you add all those fresh 'green' recruits (it's averaged). The idea being that you have your 'green troops' built around a core of grizzled veterans. Obviously you'd retain the knowledge and skills of the 'old men of the march' but it would be offset by the relative inexperience of the troops that you have added to the mix. I tend to roleplay this stuff, sending small highly experienced units into my training cities to act as 'drillmasters' for the new crop of Roman meat for the grinder. I once took a single Triarii trooper with 3 gold chevrons and sent him to Rome to 'train' my first Marian Legion. Doesn't really do anything but I like the concept.