Count for completed process are different when passing includeProcessVariables=true

Calling this API :
/history/historic-process-instances?finished=true&deleted=false&processDefinitionKey=process_def&includeProcessVariables=true&start=0&size=500&order=desc&sort=endTime

It returns total = 250 process

But Caliing the same API without includeProcessVariables=true
/history/historic-process-instances?finished=true&deleted=false&processDefinitionKey=process_def&start=0&size=500&order=desc&sort=endTime

Returns total = 400 process

There are total 400 process.Why this behaivour of returning different total count ?