As others have said the system clock is probably giving you time measurements in discrete chunks. I'm not sure why the values wouldn't be exactly 0,15,30,45 etc. This is one case where an average time can be more accurate. For example if you take 3 measurements: 0, 15, 0 then the average of 5 ms. is closer than any of the other measurements.
Being as you are performing your performance measurements on
servlets you should check out the jamon servlet filter. You can do all that you are trying to do and more with no changes to your application (just add some lines to your web.xml).
See
http://www.jamonapi.com