Why does the estimated time displayed in the task editor differ from the web interface?
Mylyn defaults to a day having 24 working hours and a week having 7 work days. If your JIRA repository uses different settings time tracking information will not show correctly when converted between Mylyn and JIRA. In order to fix this change the time tracking settings in Mylyn in the Additional Settings section of the repository properties (Window → Show View → Task Repositories) to match the server configuration. Your JIRA administrator will be able to tell you the correct settings which can be located under Administration → Time Tracking in the JIRA web interface.