When a timespan is calculated in Calculated Column of ServiceDesk or Workflow report, the resulting values shown are approximated at certain values.
For example, timespan of 1 hours and 18 minutes will be shown as "1 hour(s) and 18 minute(s)" while timespan of 2 hours and 18 minutes will be shown as "2 hour(s)".
This is working as designed. Timespan values will be converted into a descriptive phrase, omitting the smaller time units when major unit exceeds the value of 2.
Working as designed.
If this is not the desired behaviour, a report can be based on your custom SQL Query.