What would cause P6 to NOT use the default units/time when assigning a resources?

double check the default units/time

manke sure that you durations are set to fixeded duration and units/time

Assigning resources in P6 is calculated based on Resouce Calendar time period, resource default unit/time and activity type calendar time period. If activity type is Resource dependent then it will use resource calendar and if activity type is task dependent it will use task calendar.

I often see that the time period has always been neglected and forgot to change and match the calendar setting per day. This always the cause why the duration calculation is not correct including the resource assignment calculation.

Ex: 
1) Resource Calendar time period = 8hrs/day, calendar 16hr/day x 5 days/week
2) Resource Default Units/Time  = 16hrs/day
3) Task Caledar time period = 10hrs/day, calendar 8hrs/day x 5 days/week

Resource assignment calculation will be Resource units/ time divided by Resource Calendar time period multiple by Task Calendar time period = 16hrs / 8hrs x 10hrs = 20hrs/day (remaining unit/time)  x activity duration say 5 days = 100hrs

On your example it seems that the resource calendar time period is not matching the default unit/time (i.e 8h/day). Checked the resource calendar time period if it is also set 8hrs/day and matching default unit/time assigned to resources and also check the activity calendar time period if matching the calendar settings of 8hrs/day.