Calculated field script does not always return correct value

I have a simple addition calculation script to toal the RowTotal:

var proRatedPerDiem = this.getField("travel_prorated_perdiem1").value;

var lodging = this.getField("travel_lodging1").value;

var total = proRatedPerDiem + lodging ;

event.value = +total;

The "proRatedPerDiem" field is also a calculated field and when that changes, about 50% of the time, the RowTotal either does not change, or it comes up with some old value . 

I have run the javascript debugger and confirmed the script always runs, it just often doesn't return the correct value.

This is my first time working in Adobe.  Is there some weird caching or something?