Mine are always named "AW_Survival_xxx-yyyyy.jpg," where xxx increments each day and yyyy increments with each new screenshot taken. I'm not sure where the start of xxx since it doesn't match up with the day of the month or Julian date or anything, but I suspect that your cache files are somehow keeping track of this number since the numbers keep going after I log out and back in. I also move all of my screenshots to another location and never have to rename any of my files.
Try deleting your cache files and see if that fixes it.
