Don't worry about it.
Work diaries and reports present information which may be based on cached data... These pages do not necessarily show instantaneously updated information.
Sometimes it takes an hour or even longer for the reports page and work diary page to display completely up-to-date information based on all the work I have done and all the work I have manually added or deleted.
Just give it some time. I've seen it update in as little as 30 minutes and every once in awhile it takes until the next day. I would make a note of it and double check it at the end of the week during the Work Diary review, but it shouldn't take too long to update so they both match.