When running multiple instances of the same IDE, each instance maintains its own symbol database to address several reported concurrency issues. (case=12157)
this is where the extra directory has come from. If you start running multiple instances of the IDE at once further directories will be created.
But... I wasn't running 2 copies at the time. I do regularly do that but not when I noticed this. I can't be sure that there wasn't an old copy of devenv.exe running at the time. I will reinstall 1645, reboot the PC and check.
I see the same directory name when running just one instance of the IDE. The _1 seems to indicate IDE number 1. I have also generated _2 and _3 directories on one of my test systems.