Here's a screenshot having just clicked the achievements button with the two events that happened since I last clicked the button. Did 100 + 5 produce 95?
Usually such a thing means that a question you answered got deleted, and you had upvotes on your answer, hence losing reputation. Such events are not displayed in the top bar achievements dialog, however they are visible in the reputation tab of your profile, if you tick the "Show removed posts":
In the above example screenshot, this answer of mine was removed by automatic process, however there can be many reasons why question with upvoted answers can be removed, e.g. being off topic.