This does not happen all the time, but I have checked the clock and compared it to the date-last-modified of the RAMdisk image file. There have been occasions when the difference between those times was greater than 15 minutes, even though I have set a property to do an autosaving every 15 minutes.
Also, there is a very subtle bug which I can't seem to isolate: there are times when the RAMdisk image file has not been updated, even though the non-volatile source files have been updated and those updates were written to the RAMdisk in memory. I'm still trying to isolate this problem, but it is intermittent and very difficult to reproduce.
It is hard to tell what might be causing this, as the RAM Disk code writing data to image files is very straightforward and there is simply no room for this behaviour.
What can explain it though, is that the file is not updated if nothing has changed in the RAM Disk filesystem. The whole RAM Disk space is divided into 1 MB blocks and only the blocks that have changed since the last write are updated. If no blocks have changed, the file is not updated.