And I think they just get built as the orders come in, so your 1050 might follow a 200 down the line.

Somewhat true.  The variability also works better for manufacturing since some models take way more man hours to assemble at certain stations while others may take less at those stations and other models make just take more hours to assemble.

One method is hire extra employees say 10-20 while building one model for 2 months and then lay them off for 10 months until they are built again but you then have to train them again vs scatter the high assembly hours tractors thru the build schedule and only need 2-3 additionally employees year round and train them to work at multiple stations for the high assembly hour machines and they specialize in those machines and can help out where needed and fill in when someone is on vacation.  The latter method allows any given model to be run multiple times per year/month/week vs 1 time per year so you can level the workload.  Knowing there is more vacation taken May-Aug they may push the majority of the high hour models to Oct-Apr when more labor is available.

Another benefit of mixed model is the 1000s components are typically larger than those of the 200s so they take way more room than the 200s.  If running all 1000s the feeder line space could easily be 3-4x that needed for the 200s, but by mixing some of each size could be 1/2 the size if all 1000s and only 2x the size of all 200s so it results in more efficient use of building space.