Both justanothertatc and Graham Borland are correct.
Originally, FL Wun Hundred was brought in to overcome the confusion between FL110 (Flight Level wun wun zero) and FL100 (Flight Level wun zero zero).
Both of these levels are commonly used in busy TMA/high cockpit-workload environments.
I believe, (but stand to be corrected), that the 'hundred' bit was subsequently extended to the two higher levels also.
PPLs up at FL100?? - ZOOT ALLORS!!