Happy to explain, but bear with me - it is a long story. It all stems from the long period of time between the first and last Chiltern orders for 168s.
Firstly we need to establish a few facts:
The 168s were ordered in very small batches owing to the relatively small size of the Chiltern business. There was no bus industry giant behind Chiltern and although they had investment backing from Laing and 3i, the franchise bid was nevertheless financed on something of a shoestring (some of the directors in the management buy out team actually remortgaged their houses to finance the bid). So Chiltern had to order as many new trains as they could afford, build up the business, make a profit and then order more to meet demand. This affordibility relates to the lease charges by the way, not the purchase cost which was covered by Porterbrook & HSBC.
You now have to get your head around a concept that some enthusiasts seem to struggle with. That is that 168s are Turbostars, not some separate type of train. The only reason 170s were given a different numerical classification is that they have a slightly different electrical compatibility that means they can couple with Sprinter type units. 168s are electrically compatible with 165s and 166s. That is the only technical difference. They are all Turbostars. A Clubman is not a separate type of train it is a marketing name that Chiltern chose to give their Turbostars. Sorry to lay this on so thick but I do get a bit fed up of explaining this to people who insist they know better.
Chiltern got in their order for new trains very quickly. Their order for four 3-car units was the first post-privatisation order for new trains (it was later increased to five 4-car trains). The UK train building industry did not have an off the shelf product ready to put into production so Chiltern worked closely with Adtranz to specify a new DMU. Although the train was designed to Chiltern's specification Adtranz were obviously aware that further orders would come from other TOCs. In particular a swish new cab design was desirable from a marketing point of view but it required a lot of development and approval which could not be done in time to meet the Chiltern order deadline. Therefore the small initial Chiltern build were given a cab that was based on existing experience from the Networker build.
By the time Adtranz recieved their second Turbostar order (from Midland Mainline) the new cab design was ready and various other minor design changes were made - an evolutionary process that has continued throughout the whole Turbostar production run.
When Chiltern's second order for 168s was made (168106 - 168110 as 2 car units) the Turbostar specification had thus changed so the trains were built to the revised spec.
There then followed orders for a further 3 three-car units (168111 - 168113) and new centre cars for 168106 - 168110 which had previously borrowed centre cars from the 168/0s. Again, the passing of time meant that each order incorporated more detail design changes as Adtranz refined the product.
By the time the 168/2s were ordered, design evolution and a takeover of Adtranz by Bombardier had brought in more changes, for example new head/tail light clusters, revised front valance, electrical cross-feeding between cars, MITRAC diagnostic system etc.
Adtranz/Bombardier would have been neither willing nor able to build a new train to an obsolete specification so each time a new order came in for Chiltern they provided what was then the current Turbostar product, hence the varied 168 fleet.
I feel I have rambled rather a lot - hope it makes some sort of sense!