random bakcup versions being left behind

I am using the following settings for my backup method:
Incremental
Create a full version after every 6 incremental versions
Store no more than 1 recent version chains

Last night the 18th full backup (_b18_s1_v1) was created, however I noticed when looking in the destination folder that (_b16_s2_v1) and (_b11_s7_v1) still exist. The files are not terribly large, 376MB and 405MB respectively.

Any thoughts on why they remain? I ran a validation, which said it completed successfully. Is it possible that these files are there because of restoring the system at certain times?