Last 1% takes longer than the completed 99%. There are many examples to support this statement. For example progress bars we see on computers, the last period last minute, last minute of a match etc. The 90+ percent it runs fast and rest is sluggish.
In software development if you do not follow SDLC (Software Development Life Cycle) 99% complete syndrome happens. If you ask developer about the progress he/she may say development is in final stage of just 1% is left .Most probably, this last 1% will drag on to days, weeks, and even few months. Either something was wrong in the SDLC or they failed to follow up. Developers should evaluate the amount of effort needed to complete remaining tasks in hour. Usually instead of this people tend to consider the amount of work gone through against the original estimate.
If you hear someone saying that they are 99% complete. It indicates that they still have much to complete and the person has no idea when it will be completed.