calculate "around" a variable

[dedelman_clng]

I am attempting to use variables within a measure, but not getting the desired results.  I've looked around at all kinds of DAX reference material and can't figure out why this doesn't work.  I know there are other ways to accomplish why I'm trying to do, but I'd like to know specifically why this doesn't work.

Basically, I'm setting a variable and then trying to recalculate that variable within the same measure.  The recalculation doesn't work.

Data (table name is PD):

Dt	Value
1/1/2018	100
1/2/2018	200
1/3/2018	250
1/4/2018	300
1/5/2018	400

Dt has a relationship to a Date table (DateTab)

Creating a measure that finds the difference in Value from the previous day:

Delta = 
  var __TodayVal = SUM(PD[Value])
  var __PrevVal = CALCULATE(__TodayVal, PREVIOUSDAY(DateTab[Date]))
  var __Delta = __TodayVal -__PrevVal

return __Delta

Here is what I would expect

Dt	Value	Delta
1/1/2018	100
1/2/2018	200	100
1/3/2018	250	50
1/4/2018	300	50
1/5/2018	400	100

But this is what I get

I'm sure it has something to do with evaluation context inside the measure, but I'm wracking my brain to be able to explain it. I know I can work around it by creating multiple measures, but I'd like to know a concrete reason for why this can't be done in a single measure.

Thanks,

David