The reason for this is that you must request that specific field. The url will actually be something like:
http://.attask-ondemand.com/attask/api/tshet/&fields=hours:*
The reason for this is because the "hours" column is actually a collection of hours entries. You must specifically request that field in order for it to be populated. You can also see more information by going to: http://developers.attask.com/api-docs/