If I log more than one geocache per day, it sometimes occurs that the order of the logged caches in the generated HTML page changes within that day (see attached screen shot). Probably) due to that, milestones are calculated wrong. This behaviour is at least the case for geocaching.com, I cannot say anything about OpenCaching.
The order of the caches in the generated HTML page should be the same as the order they were logged on geocaching.com.
GC.com Quick View vs. geolog HTML page